WELDINGResources / Welding symbolsUpdated 29 Oct 2026

Fillet, plug, spot and seam weld symbols

Fillet welds join parts at an angle, in T, lap and corner joints, and are the most common welds on a structural drawing. Plug, slot, spot and seam welds join overlapping parts, and back welds and surfacing complete the set.

SymbolAWS A2.4ISO 2553What it is

Fillet weldA fillet weld joins two surfaces at roughly a right angle, in a T, lap or corner joint, with a weld of triangular cross-section.

Double fillet weldA fillet weld on both sides of a T joint.

Plug or slot weldA plug or slot weld fills a round hole (plug) or an elongated slot in one plate to join it to the plate underneath.

Spot weldA spot weld joins overlapping sheets at discrete points.

Seam weldA seam weld is a continuous weld along overlapping sheets, often a row of overlapping resistance spots made with roller electrodes.

Surfacing weldA surfacing weld builds up weld metal on a surface rather than joining parts: to restore dimensions, add wear resistance or apply a corrosion-resistant overlay.

Back or backing weldA back weld is a pass made on the far side of a groove weld, after the groove is welded.

Fillet, plug, spot and seam welds: AWS and ISO

The shapes match in both standards; the sizes do not. AWS gives a fillet its leg size by default. ISO 2553 writes the letter first: a for the throat thickness and z for the leg length, for example a5 or z7. Read a throat as a leg and the weld comes out about 30 percent too small; read a leg as a throat and it is about 40 percent too large.

Resistance spot and seam welds have no arrow side or other side, so the symbol is centred on the reference line in both standards.

Frequently asked questions

Which fillet, plug, spot and seam welds appear on a welding drawing?

This legend covers fillet weld, double fillet weld, plug or slot weld, spot weld, seam weld, surfacing weld, back or backing weld. Project legends may add more.

Are these symbols different in ISO and AWS?

4 of the 7 are drawn differently. Each symbol page shows both forms side by side.

Can I download these symbols?

Yes. Each symbol page has free SVG files, and the hub has printable PDF charts.

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Fillet, plug, spot and seam welds: AWS and ISO

Frequently asked questions

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