ISO 2553 vs AWS A2.4 welding symbols

Both standards use the same arrow, reference line and weld glyphs — but they place the symbols differently. Reading an American drawing with ISO habits (or the reverse) is one of the most common causes of welds ending up on the wrong side of the plate.

Same weld, two notations

ISO 2553

ISO 2553 — arrow side on the solid line, other side on the dashed line.

AWS A2.4

AWS A2.4 — arrow side below the line, other side above the line.

The differences

Aspect	ISO 2553	AWS A2.4
Reference line	Two lines: a solid reference line plus a dashed identification line.	One single solid reference line. No dashed line.
Arrow side	Symbol placed on the solid reference line.	Symbol placed below the reference line.
Other side	Symbol placed on the dashed identification line.	Symbol placed above the reference line.
Both sides	Symbol on the solid line and on the dashed line.	Symbol above and below the same line; the dashed line is never used.
Units	Millimetres and degrees.	Inches (fractions or decimals) and degrees.
Fillet weld size	Throat "a" or leg "z", e.g. a4 or z6, written before the symbol.	Leg length only, e.g. 1/4", written to the left of the symbol.
Intermittent welds	Number of welds × length (pitch), e.g. 3×50 (100).	Length − pitch, e.g. 2−5, written to the right of the symbol.
Current edition	ISO 2553:2019.	AWS A2.4:2020.

Practical advice

Always check the title block for which standard applies before interpreting a symbol.
ISO 2553 also allows a single-line (AWS-like) system, so a missing dashed line does not automatically mean the drawing is AWS.
Never mix the systems on one drawing — pick one standard and state it explicitly.