Seam Weld Symbol Explained: How to Read, Draw, and Apply It on Real Blueprints

Table of Contents

What Is a Seam Weld Symbol? A Plain-English Introduction

You’re staring at a fabrication drawing for a leak-tight fuel tank. There it is: a small circle sitting right on the reference line of a welding symbol. Is it a spot weld, a plug weld, or something else entirely? Get that circle wrong and you’ll weld a series of dots where the engineer wanted an unbroken, pressure-tight seam.

That little circle is the seam weld symbol, and reading it correctly is the difference between a tank that holds fuel and a tank that weeps at every joint.

A seam weld is exactly what it sounds like: a continuous weld run along a joint, usually a lap joint. Instead of discrete, spaced-out nuggets, the individual welds overlap each other, edge to edge, until they form one gas- and liquid-tight seam. The seam weld symbol represents that continuous or intermittent series of overlapping welds, and it’s drawn as a circle centered on the reference line of the complete welding symbol. The symbol can describe a continuous seam or an intermittent one, where short seam segments repeat along the joint. Either way, the circle stays the same; the length and pitch numbers carry the detail.

Now, a quick vocabulary cleanup. In the US, AWS A2.4:2020 is the standard that defines weld symbols, and it draws a sharp line between two easily confused terms. The weld symbol is the glyph itself, the circle. The welding symbol is the whole assembly: arrow, reference line, circle, and tail. Mix those terms up in a meeting and a detailer will wince.

Here’s the part that trips people up: that same circle is shared by spot welds, projection welds, and seam welds. The process goes in the tail, which is why a tail reading “RSEW” tells you it’s a resistance seam weld. Per ANSI’s overview of the standard, A2.4:2020 revised several symbol elements, so it pays to know which edition your shop uses.

Where You’ll See It in the Real World

Anywhere a joint has to seal: fuel tanks, radiators, HVAC ducting, appliance panels, automotive body panels, and seam-welded tube and canning lines. If the part holds liquid, gas, or pressure, there’s a good chance a seam weld is doing the sealing.

How Seam Welds Are Actually Made

The classic method is resistance seam welding (RSEW), which uses rotating roller electrodes that squeeze overlapping sheets and pass current through them. The wheel-shaped electrodes ride the joint, and the overlap between successive pulses is what creates the pressure-tight seal. Resistance welding joins metals by electrical resistance at the faying surface, with no filler metal, no shielding gas, and no arc. That’s why the process dominates high-volume canning and tube mills.

But seams aren’t only a resistance process. When a drawing calls for a full-penetration, pressure-tight seam on heavier sheet or plate, an arc process may be specified instead. Gas metal arc welding (GMAW) and submerged arc welding (SAW) are both common on tanks and vessels. Those runs depend on solid machine setup; clean contact tips and steady wire feed matter. See our guides on contact tip welding and gasless MIG welding for setup details, or robotic submerged arc welding when the seam is long and automated.

Quick Note: Read the tail first. The circle alone won’t tell you whether a joint is a spot, projection, or seam weld. The process letter code in the tail (RSEW, GMAW, SAW) is what settles it.

This is a skill a lot of people rely on. The American Welding Society counts roughly 771,000 U.S. welding professionals as of 2025 and projects about 320,500 new professionals will be needed by 2029. That’s a lot of hands reading the same small circle.

What’s Ahead

Over the next eight sections we’ll dissect the full welding symbol piece by piece, decode that circle against spot, projection, and plug welds, walk through sizing and pitch, compare the US and ISO systems, flag the mistakes that cost shops real money, and finish with a worked example you can read line by line. If you want the hands-on foundation underneath all of it, start with our guide to welding practice, which covers the fundamentals that make reading these symbols second nature.

Anatomy of the Welding Symbol: Arrow, Reference Line, Circle, and Tail

Every welding symbol you’ll ever read is built from the same handful of parts. Learn the pieces once and any drawing becomes readable, whether it’s a simple bracket or a pressure vessel with a dozen callouts. Per ISO 2553, the welding symbol is made of an arrow line, a reference line, and a tail. Everything else hangs off those three.

Before the parts, one wording trap. AWS A2.4 distinguishes between a weld symbol and a welding symbol. The weld symbol is just the glyph, like the little circle or the V-shape. The welding symbol is the complete assembly of lines, glyph, and notes. When somebody says “read the weld symbol,” they usually mean the whole package.

The Arrow Line

The arrow line is the leader that points at the joint. Its tip touches the edge, seam, or location where the weld goes. The arrow can have a break in it, and it can point at either member of a joint. That’s how the drawing tells you which face you’re dealing with.

On a fillet or groove weld, the arrow’s touch point matters because it defines the arrow side and the other side. The glyph drawn below the reference line applies to the arrow side; the glyph drawn above applies to the other side. This is the classic rule beginners trip over.

The Reference Line

The reference line is the horizontal spine of the symbol. The elementary weld symbol sits on it, the tail extends from its right end, and the arrow meets it at the left. Here’s the part that matters for consistency:

Quick Note: The reference line is always drawn horizontal, parallel to the bottom edge of the drawing, so the symbol reads left to right. Never rotate it to follow a slanted joint. The arrow bends to reach the joint; the reference line stays level.

That single convention is why weld symbols look identical across a shop drawing, a CAD model, and a hand sketch. It keeps the reading direction constant even when the part is drawn at 45 degrees.

The Circle at the Intersection

For a seam weld symbol, a small circle is drawn at the junction where the arrow line meets the reference line. That circle is not a separate weld; it’s a modifier that tells you the weld runs continuously along the joint rather than being a series of discrete dots.

This is where the arrow’s “side” loses its meaning. A seam weld (and a spot weld) fuses overlapped members at the faying surface, the interface trapped between the two sheets. The weld happens between the parts, not on an exposed face. So there’s no meaningful arrow side versus other side the way there is with a fillet laid in a corner. The circle at the intersection signals exactly that: the weld is internal to the lap, in line with the reference line, not deposited on a visible face. The same positioning logic is used on spot welds, which you’ll see contrasted in spot wd cutters and their symbols.

The Tail

The tail is the short fork at the far right of the reference line. It’s optional, because not every weld needs a process note. When it’s used, the tail carries the welding process, a reference to a specification, or a general note like “grind flush.” A tail with no text can also mean “use the process specified elsewhere.”

Supplementary Symbols That Travel With a Seam

Supplementary symbols modify the elementary symbol but never replace it. Three pair commonly with a seam weld:

Weld-all-around: a small circle at the elbow where the arrow meets the reference line. It tells you the weld continues around the entire joint perimeter.
Field weld: a flag on the arrow-to-reference elbow. It means the weld is made at the site rather than in the shop, useful when a tank or frame is fabricated in sections.
Melt-through: a symbol indicating the weld penetrates through the backing or root to the far side.

Notice the elbow gets crowded fast. The all-around circle, the field flag, and the seam circle all live near that junction, so on a busy drawing it pays to know which is which. Recognizing these modifiers is a core prep skill; if you want to sharpen that eye before hitting real drawings, structured welding practice helps you translate symbols into actual arcs.

Get the four core parts down, arrow, reference line, elementary symbol, and tail, and you can decode the basic callout. Add the seam circle, the all-around and field flags, and the numbering rules later. The structure never changes; only the glyphs do.

Decoding the Circle: Seam vs Spot, Projection, and Plug Weld Symbols

Nothing trips up a new reader faster than the little circle on a welding symbol. It shows up on spot welds, seam welds, projection welds, plug welds, and slot welds — five different joints sharing one family of shapes. Get the details right and the drawing reads itself. Miss them and you’ll build the wrong thing entirely.

Here’s the good news: each variant adds or removes one small element. Once you know which element, you can tell them apart at a glance — a welding symbols chart makes this easy to spot.

The Circle Family: Three Symbols, Three Meanings

Three symbols in the AWS A2.4 standard start with a circle placed at the junction of the arrow and reference line:

Symbol	Shape	Meaning
Spot weld	Circle only	Discrete round nugget at one location
Seam weld	Circle with two horizontal lines through it	Continuous overlapping line of welds
Projection weld	Circle only, process in the tail	Nugget formed at a raised dimple or projection

All three belong to the resistance welding family, which includes spot, seam, projection, upset butt, and flash welding. The shape tells you what geometry you’re producing; the tail tells you which process makes it.

Seam vs Spot: The Extended Lines Are Everything

The seam weld symbol is commonly drawn as a circle with two short parallel lines running left and right through it — often described simply as two horizontal lines through the circle. Those lines are the whole point. They signal a continuous, overlapping line of welds rather than a series of discrete nuggets.

Compare the two side by side:

Spot weld: a plain circle. One nugget, one location, done. Think of a dot.
Seam weld: a circle crossed by two lines. A wheel or roller travels the joint, producing overlapping nuggets that form a pressure-tight seam. Think of a line.

That difference matters on a leak-tight fuel tank. A spot weld symbol gives you a string of dots with gaps between them — fine for holding sheet metal panels together, but porous under pressure. The seam weld symbol demands an unbroken path. That’s why the reference line, circle, and tail matter so much here: swap one element and you change the physical result.

Because spot and seam welds share the same circular family, the tail is what removes all doubt. It specifies the exact process, such as RSW for resistance spot welding or RSEW for resistance seam welding. Read the tail before you commit.

Quick Note: If you’re working with spot-welded sheet metal assemblies, a spot weld cutter bit drills out the nugget for repair without destroying the base metal — the practical counterpart to reading the symbol in the first place.

Projection Welds: Process in the Tail

A projection weld symbol looks identical to a spot weld at first glance — just a circle. The difference lives in the tail and in how the part is prepared. Projections (small raised dimples, embosses, or formed points) are created beforehand. The machine then presses the parts together and passes current through those points, collapsing each projection into a nugget. That’s why the memory aid works: the projection is drawn by the machine, not the welder’s hand. On the symbol, you’ll find the process named in the tail, often with a size callout for the projection.

Plug, Slot, and Fillet Welds: Different Shapes Entirely

Plug and slot welds do not use the seam weld’s line-through-circle configuration. Instead, they combine a rectangle or oblong shape with a circle, plus a size, and frequently a spacing or pitch value.

Plug weld: a rectangle with a circle inside represents a hole filled with weld metal, joining a top plate to a bottom plate.
Slot weld: an elongated oblong (slot) with a circle, used for a longer hole filled with weld.
Fillet weld: a right triangle — no circle at all. It joins two surfaces at roughly right angles and is the most common symbol you’ll see.

If you see a triangle, you’re in fillet territory. If you see a circle inside a rectangle or slot, you’re looking at plug or slot. Only the lone circle with two lines through it means seam.

Three Memory Aids That Stick

Spot = dot. One nugget, one place.
Seam = line. The two lines through the circle mean a continuous run.
Projection = drawn by the machine. The part is formed; the process hides in the tail.

Practice on real drawings and the distinctions become automatic. It’s part of the broader skill set covered in our guide to welding practice — reading symbols is a muscle you train, not a fact you memorize once.

Sizing a Seam Weld: Size, Strength, Length, Pitch, and Number on the Symbol

A seam weld symbol never travels alone. Everything that tells the welder how big the weld is, how long it runs, and how often it repeats hangs off the reference line in a strict left-to-right order. Place those numbers correctly and the symbol reads like a sentence. Place them wrong and the shop builds the wrong joint. Here’s how the dimensioning conventions actually work.

What Goes on the Left: Size or Strength

The reference line is the horizontal spine of the symbol, and the space to its left is reserved for one value: the weld’s size. On a continuous seam that number is the leg or throat dimension, in inches or millimeters.

For decades, that same slot did double duty for spot, seam, and projection welds, carrying a shear strength value like “10 000 N” instead of a size. That changed with AWS A2.4:2020. A value placed to the left of a spot or seam weld symbol now designates only the weld size. Strength must be written as a note in the tail. The AWS A2.4 standard spells this out, and this ANSI summary of the change is a fast sanity check when you’re updating old prints.

That distinction matters on legacy drawings. A 2007-era symbol could legitimately show a strength figure in the left position; the 2020 edition removed the ambiguity. One position, one meaning. On pipe and pressure work, that same slot also decides whether the seam is sized by leg or by throat, so confirm which the designer intended before you set your machine, since the two call for different welding practice.

Quick Note: If a left-hand number has units of force (N, lbf) rather than length (mm, in.), treat the drawing as outdated and confirm the intent before welding.

What Goes on the Right: Length and Pitch

Length lives to the right of the weld symbol, and pitch — the center-to-center spacing between weld segments — goes further right still. For an intermittent seam, the two are combined into a length-pitch format separated by a hyphen: 2-6 means a 2 in. long weld every 6 in. on center.

Read it as one phrase: make the weld, skip ahead, make the next one, repeat. The pitch is always measured between matching points on adjacent segments, not from the end of one weld to the start of the next. If only a single length is given with no pitch, the seam is continuous.

Counting the Welds

When the same seam repeats across a part, the count goes in parentheses near the symbol — for example, (4) tells the welder to make four identical segments. On some drawings you’ll instead see a plain count or a multiplier. Either way, the number describes how many welds, while length and pitch describe each one. A seam drawn as a single unbroken line with no pitch and no parentheses is simply continuous — the default when the designer wants full coverage.

Worked Example: Reading an ‘X’ Seam Weld Left to Right

Picture a seam weld symbol with a size callout of 1/4 on the left, 2-6 on the right, and a tail note reading “SEAM WELD, STRENGTH PER NOTE 3.”

Read it left to right: the 1/4 is the weld size (1/4 in. throat). The 2-6 says each segment is 2 in. long, repeating every 6 in. on center. The tail carries the strength requirement because the 2020 convention no longer allows it beside the symbol. Nothing in the left slot says anything about strength. That’s the whole point.

Intermittent vs Continuous Seams

Intermittent, or stitch, seams cut heat input and distortion, which is why they show up on sheet-metal ducting, panels, and stiffeners where a full-length weld would warp the part. The trade-off: a stitched seam is not leak-tight. Stitch spacing also governs how much the plate is free to move between passes, so a tight pitch means a stiffer, stronger assembly.

A continuous seam, by contrast, is the choice whenever containment matters — pressure vessels, tanks, pipe, and fittings that must hold fluid or gas. That’s where leak-tight tube and pipe work like weld tube fittings demands full-length coverage. High-deposition automated processes such as robotic submerged arc welding are often matched to long continuous seams where even, uninterrupted travel matters. On the standard side, ISO 2553:2019 devotes clause 5.9 specifically to dimensioning seam welds, so check that clause before assuming the AWS layout applies to your metric drawing.

Seam Weld Symbol in Practice: Joints, Processes, and Where It Appears

The seam weld symbol almost always points at one geometry: the lap joint. Two sheets or plates overlap, the arrow touches the exposed edge, and the weld runs the full length where the members meet. Because the symbol sits on the reference line with that small circle, the welder knows the finished joint must be continuous — not a scatter of individual dots that leave gaps between them. That single geometric assumption shapes every decision that follows: process, machine, travel speed, and inspection.

What separates a seam weld from an ordinary fillet on a lap is where the metal fuses. The weld forms on the faying surface, the hidden interface between the two overlapping members. In resistance seam welding, electrode wheels squeeze the stack from both sides while current flows through the contact resistance at that interface, so fusion happens inside the joint rather than on the visible outer edge. In arc-based seams, the torch or gun runs the exposed edge of the lap, but the goal is still full fusion along the overlap so no capillary gap survives to leak.

Why Leak-Tightness Drives the Design

Most seam weld symbols exist for one reason: the joint has to hold pressure or keep fluid in. A tank, radiator, or duct that weeps at the seam has failed, no matter how strong the weld is elsewhere. That is why the symbol calls for a continuous seam rather than intermittent welds. An unbroken bead leaves no pinhole path for gas or liquid to escape, which is exactly the point. Seam welding is favored wherever a leak-tight lap joint is required, and tanks, radiators, and ducting are the classic examples.

Quick Note: The symbol is process-agnostic. It tells you the joint must be continuous, not which machine makes it. Choosing RSEW, GMAW, or SAW is a separate decision driven by thickness, volume, and access.

Processes That Produce a Seam Weld

The process list follows material thickness and production volume:

Resistance seam welding (RSEW) uses rotating wheel or roller electrodes to make a continuous series of overlapping spot welds. As the wheels roll and the current pulses, each nugget overlaps the last until the seam is effectively solid. Knurled or profiled wheels concentrate current on coated or oxidized stock. This is the fast, automated route for thin sheet at high volume.
GMAW and FCAW cover small-batch work, field repairs, and awkward geometry where a wheel can’t reach. For dirty or windy outdoor conditions, a gasless flux-cored setup keeps the seam moving; gasless MIG welding sidesteps shielding-gas problems entirely and is a common field-repair choice.
Submerged arc welding (SAW) is the high-deposition workhorse for long, continuous seams on heavier plate. It buries the arc under granular flux, lays down a large amount of metal per pass, and is usually run automatically. Robotic submerged arc welding pushes that further for repetitive tank and vessel shells where the same seam repeats hundreds of times.
Laser and hybrid laser-arc welding run at high speed on thin gauge, producing narrow, deep, low-distortion seams. They show up in tailored blanks and precision enclosures where heat input must stay small.

Where the Symbol Shows Up on Real Drawings

Fuel tanks and pressure vessels — shell-to-end-cap seams that must stay sealed under pressure.
Radiators and heat exchangers — tube-to-header seams that hold coolant for years.
Transformer cases and electrical enclosures — oil-tight, weather-tight boxes.
HVAC spiral duct — the longitudinal lock seam becomes a welded seam; metal rolling and welding often run together in this work.
Food-grade stainless vessels — continuous, cleanable seams that can be sanitized without harboring bacteria.
Automotive exhausts and body assemblies — muffler shells, catalytic converter cans, and panel joins.

In automotive and appliance plants the picture shifts slightly. There, body panels and appliance enclosures are heavily resistance seam and spot welded in high-volume production. A drawing may call for a “rolled seam” built from a line of overlapping spot welds, and the seam weld symbol communicates that those individual welds must overlap enough to read as one continuous, sealed line. Equipment for this kind of work is trained and maintained with real discipline, because a single worn wheel can open a leak across an entire production run.

Read the symbol, then match it to the process your shop actually owns. The geometry and the leak-tight requirement stay the same; only the machine changes.

AWS A2.4 vs ISO 2553: Two Systems, One Symbol, Different Rules

Cross a border with a welding drawing and the same circle-on-a-line symbol can suddenly mean something slightly different. Two standards dominate global fabrication: AWS A2.4 in the United States and ISO 2553 internationally. Both define a seam weld symbol, but they organize the reference line, place dimensions, and label processes in ways that don’t always translate one-to-one. Knowing which standard governs a drawing, and spotting the differences before you strike an arc, is a core blueprint-reading skill.

AWS A2.4: One Reference Line, Two Sides

The American system is built around a single solid reference line. Symbols drawn below that line describe the arrow-side weld; symbols above it describe the other side. There’s no dashed line to interpret, which makes A2.4 diagrams fast to read once you memorize the convention. The AWS A2.4 standard also lets the tail carry process and consumable notes, so a single symbol can encode both geometry and method.

The arrow points to the joint, and the side the arrow touches is the arrow side. From there, position on the reference line does all the talking.

Because A2.4 is a US national standard, drawings from American fabricators, schools, and shops almost always follow it. If you trained on A2.4, the ISO style can look almost familiar, which is exactly where misreads happen.

ISO 2553: System A and System B

ISO 2553:2019 is more flexible, and that flexibility trips people up. It recognizes two reference-line systems. System A uses a dual reference line (a continuous line plus a dashed line), inherited from ISO 2553:1992. System B uses a single reference line, based on Pacific Rim practice.

System B exists largely because Pacific Rim companies had long used a single-line convention, and ISO wanted a path for them to stay compliant without redrawing legacy work. System A remains closer to European practice, with the dashed line separating the two sides of the joint.

In System A, a symbol on the continuous line means the arrow-side weld; a symbol on the dashed line means the other side. In System B, the placement flips to the single-line logic familiar to many Asian and Pacific fabricators. The ISO 2553:2019 text lays out both, and clause 5.9 covers dimensioning of seam welds specifically.

Feature	AWS A2.4	ISO 2553:2019
Reference line	Single solid line	System A: dual line; System B: single line
Arrow-side weld	Below the line	System A: continuous line; System B: above the line
Other-side weld	Above the line	System A: dashed line; System B: below the line
Seam weld dimensions	A2.4 convention	Clause 5.9 convention
Process in the tail	Letters or abbreviations	Numbers per ISO 4063

Quick Note: The first thing to check on any ISO drawing is whether it declares System A or System B. Same symbol, same clause number, completely different reference-line layout.

Where Dimensions and Suffixes Differ

Both standards size a seam weld, but they don’t place the numbers identically. ISO leans on convention suffixes and designators tied to the symbol type, while A2.4 keeps its own arrangement. Reading a size off the wrong side of a symbol, or assuming an ISO leg length means what an AWS leg length means, can quietly change the weld you build.

One practical habit: before applying any number, confirm which line it attaches to and which side of the joint that line represents. The suffix and the line position work together; neither is meaningful alone.

The Tail Tells You the Process

The tail is where the two systems really split. ISO uses process reference numbers under ISO 4063 — 131 for MIG/MAG, 141 for TIG, and so on. AWS practice commonly uses letters or abbreviations instead. Same intent, different shorthand, which is why a tail that reads “141” means nothing to a welder trained only on US drawings.

Don’t Mix Standards on One Drawing

Mixing the two on a single project invites conflicting interpretation. A single drawing should reference one symbolic standard, and that reference belongs in the title block or a general note. Patchwork drawings where some symbols follow A2.4 and others follow ISO 2553 are a recipe for a seam welded in the wrong place or to the wrong size. Good welding practice starts with a drawing everyone reads the same way, and standard discipline is part of that.

Common Mistakes When Reading and Drawing the Seam Weld Symbol

Even seasoned designers and welders trip over the same handful of errors, and most of them are small omissions rather than exotic misunderstandings. Each one turns a clear instruction into a guessing game on the shop floor. Here are the mistakes that show up again and again.

Mixing Up Seam and Spot Weld Symbols

The single most common blunder is treating a seam weld symbol like a spot weld symbol. Both use a circle, but the seam symbol also carries two parallel lines running along the reference line to signal a continuous weld. Leave those lines off and you have just told the welder to make a row of dots. Add them where a spot weld was intended and you have ordered a full continuous seam nobody asked for. Misinterpretation of resistance welding symbols on engineering drawings is cited as a common cause of poor weld quality and rework, and this confusion sits right at the top of the list.

Putting Shear Strength on the Left

In older AWS practice, a value to the left of the reference line could represent shear strength. That changed. Weld size and strength are now separated under AWS A2.4:2020, so only size belongs on the left while strength moves elsewhere. Drop a shear strength number there today and the shop reads it as a size, then builds a weld with the wrong cross-section. The revision exists precisely to kill that ambiguity, so work from the current AWS A2.4 symbol standard rather than an old drawing set.

Leaving the Tail Empty

An empty tail is lazy drafting. Without a process reference, nobody knows whether you mean RSEW, GMAW, or laser. The welder guesses, the estimator guesses, and the wrong machine gets booked for the job. Fill the tail with the process and any supporting notes.

Over-Specifying a Continuous Seam

Specifying a continuous seam when an intermittent stitch would meet the load wastes filler, shielding gas, and labor, and it ties up a machine that could be doing other work. Over-specification feels safe, but it raises cost and heat input. Match the weld type to the actual load case instead.

Misplacing Length and Pitch

Intermittent welds depend on two numbers, the length of each weld and the pitch between them, placed in the correct order and on the correct side of the reference line. Swap them or move them across the line and the welder produces the wrong segment length or spacing. On a long seam, a small drafting slip becomes a serious structural error.

Ignoring All-Around and Field Weld Flags

The all-around symbol and the field weld flag change where and by whom the weld is made. Miss them and a fabricator may wrap a weld that should have stopped at a joint, or weld in the shop what was meant to be field-fitted on site. Those flags carry real logistical weight, so coordinate with the shop early whenever you are unsure.

Writing a Size the Shop Cannot Produce

Calling out a seam size that available electrodes or equipment cannot reach is a drawing-room fantasy. Resource, process, and equipment mismatches, not equipment failure, are often at the root of resistance weld quality problems, per AWS industry guidance. If the electrode cannot deliver the specified nugget or reach the joint, the drawing sets the job up to fail.

The Field Error That Causes Leaks

This is the expensive one. A discontinuous spot-welded joint where a continuous seam is specified typically fails leak and pressure testing. The part looks welded, passes a quick visual, then weeps under load. A leaking tank rarely shows its bad welds on the outside. Detection comes down to dye penetrant, a pressure test, or ultrasonic inspection, each able to reveal the gaps a spot pattern leaves behind. When leaks surface, they often trace back to the same fusion and gap issues seen in overlap welding defects, and they demand the same disciplined inspection and repair.

Quick Note: Before a drawing leaves your desk, read the symbol out loud as a sentence: process, size, weld type, length and pitch, all-around, field. If any part does not make sense, the shop will not be able to build it correctly.

Practical Tips, Checklists, and a Worked Reading Example

Reading a seam weld symbol is a skill, not a talent. Once you build a repeatable method, the symbol stops being a puzzle and turns into a set of clear shop-floor instructions. The goal is simple: translate a few strokes of ink into exactly what the welder must build, with nothing left to guesswork. Here’s how to handle it every time, from first glance to final inspection.

The Left-to-Right Reading Method

Work through the symbol in a fixed order so you never skip an element. Jumping around the drawing is how dimensions get missed and flags get overlooked. The sequence below catches most misreads before they reach the floor:

Identify the elementary symbol. Confirm the circle sits on the reference line, not the arrow. That circle is the seam weld marker.
Check the arrow and the other side. The side the arrow points to gets the symbol on the reference line; the opposite side uses the symbol below the line.
Read the left dimension. That number is the size (weld width or penetration), shown before the symbol.
Read the right dimensions. Length and pitch go after the symbol, telling you how long each nugget or overlap is and how far apart they sit.
Check the tail. Process codes (like GTAW or ERW) and reference notes live here, along with any supplementary details.
Look for flags. An all-around circle means weld the entire joint; a field flag means weld it on site.

Quick Note: A structured left-to-right reading order — symbol, dimensions, tail — prevents most misreads. Train your eye to flow the same way on every drawing.

Drawing-Review Checklist for Detailers and QA

Is the circle correctly placed relative to the reference line and arrow?
Do size, length, and pitch match the joint design and load requirement?
Is the process code in the tail consistent with the specified base metal?
Are all-around and field flags shown only where actually required?
Does the drawing note the standard (AWS A2.4 or ISO 2553)? The two systems place dimensions differently, so mixing them causes errors.
Has a second reader verified the symbol against the weld schedule?

Run this list on every drawing before it leaves the detail office. A two-minute check upstream is far cheaper than a rework or a rejected part downstream.

Worked Example 1: A Fuel Tank Drawing

Picture a lap joint on 1.5 mm stainless with a symbol reading 3–30/50 on the arrow side, circle on the line, tail marked “ERW.” Decode it step by step: 3 mm nugget size, 30 mm weld length, 50 mm pitch (center-to-center), resistance seam weld. On the floor that means a wheel-type seam welder with a copper electrode wheel matched to the radius, electrode force set to the schedule, and travel speed synchronized so overlaps stay consistent. Verify nugget spacing against the pitch, because a drifting pitch usually signals current or speed trouble. Because this is a leak-tight application, plan for pressure testing and dye penetrant on the finished seam before it ships.

Worked Example 2: An HVAC Duct Drawing

Now a 1 mm galvanized duct with a seam symbol sized 2 mm, no length or pitch (continuous seam), tail reading “GMAW.” Continuous means an unbroken run, so set up a wire welder with the right good and bad MIG welds reference in mind — watch for cold lap and inconsistent bead width along the seam. Travel speed and current must hold steady for a gas-tight joint, since a single slow patch can burn through the thin galvanized sheet. Confirm shop conditions first; a well-planned layout keeps airflow, fume control, and power where you need them, as covered in our guide to welding facilities.

Inspecting the Finished Seam

Verification depends on the joint’s purpose. Leak-tight seams are confirmed with hydrostatic or pneumatic pressure testing, dye penetrant, or ultrasonic inspection. For resistance seam welds, peel and macro-etch tests on coupons are the standard destructive checks — they reveal nugget size and penetration that a surface look can’t. Welders running these tests should never skip eye protection; see our guide to protective eyewear for welding. Detailers who want formal grounding can pursue AWS’s Certified Resistance Welding Technician (CRWT) credential, which covers resistance welding symbols and drawing interpretation.

Build the reading habit, verify on coupons, and the seam symbol becomes second nature.

Conclusion: Why Getting the Seam Weld Symbol Right Matters

It’s a tiny circle with two lines through it, sitting on a reference line barely wider than a pencil stroke. Yet that circle decides whether the part in front of you becomes a pressure-tight vessel or a pile of scrap. It’s the difference between a $50 test coupon and a fuel tank that fails hydrostatic testing after hours of fit-up, welding, and finishing. Get it right, and the drawing reads like a sentence. Get it wrong, and you’ll spend a week explaining why the seam that should have been continuous came out as a row of dots.

That’s the whole case for symbol literacy in one image. A seam weld is a continuous, leak-tight joint, and its symbol must say so unambiguously. A spot weld is a discrete nugget, and its symbol must say that. Two little circles, two completely different welds, and a fabricator’s livelihood riding on which one the engineer actually drew.

The Rules Worth Remembering

Everything in this guide boils down to a short checklist you can carry in your head:

The seam weld symbol is a circle with two parallel lines running through the reference line. No lines, and it’s a spot or projection weld.
Size sits to the left of the symbol; length and pitch sit to the right. Length tells you how long each segment is; pitch gives the center-to-center spacing when the seam is intermittent.
The process goes in the tail — GMAW, SAW, RSW, whatever the engineer specifies. The tail isn’t decorative; it dictates your machine, consumables, and setup.
AWS A2.4:2020 separated weld size and strength for spot and seam welds, cleaning up years of ambiguity and giving fabricators an explicit number to hit rather than an inference to guess at.
ISO 2553 splits into System A and System B, and the two don’t read the same way. Never assume a drawing follows the standard you’re used to.

That last point trips up more people than any other. A symbol interpreted under the wrong system can flip which side of the joint gets welded or point the size arrow at the wrong dimension entirely. When in doubt, check the drawing’s title block and the referenced standard before you strike an arc.

Quick Note: Ambiguity on a drawing isn’t just an annoyance — it’s a rework generator. Clear, standard-compliant welding symbols reduce rework, scrap, and project delays by removing the guesswork that causes them.

Why This Matters More Than Ever

The workforce context makes correct documentation urgent, not academic. According to AWS welding workforce data, the U.S. will need roughly 320,500 new welding professionals by 2029 — about 80,000 openings a year. On top of that, more than 157,000 welding professionals are approaching retirement age. Every experienced eye that leaves the shop takes hard-won symbol intuition with it. What remains has to be the drawing itself: unambiguous, standard-compliant, and readable by the next person at the table.

When a shop runs lean and hires fast, the drawing becomes the teacher. A seam weld symbol that clearly states size, length, pitch, and process lets a newer welder produce a correct part on the first pass. A sloppy one guarantees phone calls, rework, and delayed deliveries — the exact costs lean operations can’t absorb. The symbol, small as it is, protects quality on the floor and the schedule in the office.

What to Do Next

Don’t just read about symbols — read them in the wild. Pull a real fabrication drawing, cover the title block, and decode every symbol before checking your work. Then cross-check your reading against the governing standard: AWS A2.4:2020 for inch-based work or ISO 2553 where that system applies. Keep both within arm’s reach of the bench, because the answer to a late-night argument is always on the page.

Build the habit alongside your torch time. Honing your hands through deliberate welding practice matters, but so does training the eye that reads the paper before the metal ever hits the table. And when you’re laying out a shop or setting up a bay where these drawings get built, a well-planned workspace — covered in our guide to welding facilities — keeps reference materials, standards, and clean communication where they belong.

Read the circle. Respect the lines. Tape the standard to the wall. That tiny mark is the difference between passing inspection and starting over.

Frequently Asked Questions

What does a seam weld symbol look like?

It is drawn as a circle placed on the reference line of the welding symbol, with two short parallel lines extending through the circle to the left and right. That line-through-circle mark distinguishes it from a plain spot weld circle and signals a continuous or intermittent line of overlapping welds along a lap joint.

What is the difference between a seam weld symbol and a spot weld symbol?

Both use a circle on the reference line, but the seam weld symbol adds two horizontal lines through the circle to indicate a continuous series of overlapping welds. A spot weld symbol is just the circle, indicating discrete weld nuggets. The process is usually named in the tail, such as RSW for spot or RSEW for seam.

Where do you put the size and length dimensions on a seam weld symbol?

Under AWS A2.4, the size (or in older practice, strength) goes to the left of the symbol, and the length and pitch go to the right. In AWS A2.4:2020, a value to the left of a spot or seam weld symbol designates only the weld size; strength must be given as a note in the tail.

Why did AWS change the seam weld size and strength rule in A2.4:2020?

The 2020 edition modified the convention so that the dimension to the left of a spot or seam weld symbol designates size only. Any required strength value must now be specified as a note in the tail, removing the ambiguity that existed when the same location could mean either size or strength.

What process is normally used to make a seam weld?

Resistance seam welding (RSEW) is the classic process, using rotating wheel electrodes to produce overlapping nuggets in sheet metal. Seam welds can also be made with arc processes such as GMAW, FCAW, and SAW, or with laser welding for high-speed thin-gauge work, depending on the application.

How is the seam weld symbol different under ISO 2553?

ISO 2553:2019 also defines a seam weld symbol, but it recognizes two reference-line systems: System A (dual reference line) and System B (single reference line). Dimensioning follows its own clauses, including clause 5.9 for seam welds, and process information is typically shown using ISO 4063 process numbers in the tail.

Can a seam weld symbol call for an intermittent weld?

Yes. Intermittent seam welds are common when leak-tightness is not required, and they are dimensioned using length and pitch notation to the right of the symbol. Continuous seams are used when the joint must contain fluid or gas, such as fuel tanks and pressure vessels.

How do you inspect a seam weld for quality?

Leak-tight seams are typically verified with pneumatic or hydrostatic pressure testing, dye penetrant inspection, or ultrasonic testing. Destructive checks such as peel tests and macro-etch examination of coupons are used to confirm nugget size, overlap, and penetration in the weld schedule.

Why do so many drawings get the seam weld symbol wrong?

Because spot, seam, and projection welds all share a circular symbol family, and their differences are subtle. Combined with the 2020 AWS size-versus-strength change and the AWS/ISO split, small drafting or reading errors are common, and misinterpretation of resistance welding symbols is a documented cause of rework.