Weld symbol reference

Below the reference line means the arrow side; above it means the other side. A fillet-weld triangle under the line with “6” beside it calls for a 6 mm fillet on the joint face the arrow touches. This page lists the weld symbols you’ll meet on fabrication drawings — fillet, the groove family (square, V, bevel, U, J), plug/slot, and spot — with what each one means and where it’s typically used.

Symbol conventions come from AWS A2.4 in US practice and ISO 2553 internationally; the systems are similar but not identical, so the drawing’s referenced standard governs the fine print.

AWS symbol	ISO 2553 symbol	Type	Description
triangle	Same triangle (ISO 2553 No. 10); differs only in how throat/leg sizes are dimensioned	Fillet	Fillet weld; joins surfaces at approximately 90 degrees; the most common weld type
V	Same V (ISO 2553 No. 2); a plain ISO butt symbol is full-penetration unless dimensioned otherwise	Groove V	Single-V groove weld; V-shaped edge preparation; penetration follows the dimensions and notes (not implied full-penetration)
U	Same U (ISO 2553 No. 6)	Groove U	U-groove weld; radius-bottom groove weld for thick sections; reduces weld volume vs V-groove
J	Same J (ISO 2553 No. 7); a broken (bent) arrow points at the prepared member in both standards	Groove J	J-groove weld; single-sided radius groove; used for thick plate T-joints and corners
parallel lines	Same two short parallel vertical lines (ISO 2553 No. 1)	Groove Square	Square groove weld; no edge preparation; for thin materials up to ~6mm
half-V	Same half-V (ISO 2553 No. 4); perpendicular leg drawn on the left, with the broken-arrow rule	Groove Bevel	Bevel groove weld; single-sided V preparation; common for T-joints
rectangle	Same open-bottom rectangle (ISO 2553 No. 11); circular holes covered by ISO clause 5.6	Plug	Plug weld; circular hole in one member filled with weld to join to underlying member
rectangle	No separate ISO symbol — ISO uses the No. 11 plug symbol for elongated holes (clause 5.7); AWS gives slot its own column	Slot	Slot weld; elongated hole filled with weld; similar to plug but non-circular
circle	Same circle, but ISO splits it by process (No. 12.1 resistance / 12.2 projection / 13 fusion) using symbol placement	Spot	Spot weld; localized fusion at discrete points; common in sheet metal (welding process stated in the tail)
circle with two parallel lines through it	Same circle-with-two-lines glyph, but ISO splits it by process (No. 14 resistance / 15 fusion) using placement	Seam	Seam weld; continuous weld along a joint for leak-tight sheet metal joints (welding process stated in the tail)
semicircle	Not in ISO Table 1 — ISO Table 3 No. 5 uses one supplementary symbol for a back run or backing weld; the specified run sequence (e.g., reference lines, tail note, or WPS) distinguishes them, not the glyph	Back	Back run/backing weld; deposited on the root side after the groove weld or used as backing before it, according to the specified sequence
series of humps	Same double-hump glyph, named overlay in ISO (No. 21, clause 5.12)	Surfacing	Surfacing weld; buildup/overlay for wear resistance or dimensional restoration
two parallel vertical lines with flanged/curled tops	Similar edge symbol (ISO No. 19); ISO also keeps flanged-butt/-corner (No. 20) and stake (No. 22) symbols that AWS retired	Edge	Edge weld; weld on edge joint where edges of two members are approximately parallel
circle with an X (⊗); arrow-side only	Same circle-with-X (ISO 2553 No. 16); shown arrow-side only, diameter to the left	Stud	Stud weld; stud fastener welded to base metal by electric arc or resistance
flared V	Same flare-V (ISO 2553 No. 8); always dimensioned per ISO clause 5.4.4	Flare V	Flare-V groove weld; radius-to-radius joint between two curved surfaces (e.g., tubes)
flared half-V	Same flare-bevel (ISO 2553 No. 9); always dimensioned	Flare Bevel	Flare-bevel groove weld; flat surface to curved surface joint (e.g., tube to plate)

Arrow side vs other side

In short: a weld symbol below the reference line means weld the arrow side; above the line means the other side — no matter which way the arrow points.

A welding symbol is an instruction set anchored to the joint by two lines: an arrow pointing at the joint and a horizontal reference line that carries the weld symbols, dimensions, and a tail for process notes. Everything hinges on which side of the joint each symbol applies to. Under AWS A2.4, a symbol below the reference line means weld the side the arrow touches (the arrow side); a symbol above the line means the opposite side (the other side), regardless of arrow direction.

ISO 2553 System A answers the same question with a doubled reference line — a solid line paired with a dashed identification line. A symbol on the solid line is an arrow-side weld; a symbol on the dashed line is an other-side weld, wherever that dashed line is drawn. When a weld is symmetrical about the joint (identical on both sides, such as a double fillet), the dashed line is deleted. ISO 2553:2019 also defines System B, a single-reference-line method whose placement logic matches AWS. A drawing must declare which system it uses, and the two are never mixed.

Arrow line — points at the joint; a broken (bent) arrow points at the member to be prepared.
Reference line — the horizontal carrier for the symbols and dimensions.
Symbol below the line (AWS / ISO System B) = arrow side.
Symbol above the line = other side.
ISO System A — solid line = arrow side; dashed identification line = other side.
Symmetrical double-sided welds (System A) — the dashed line is omitted.
Tail — process references (ISO 4063 numbers or AWS letters) and spec / WPS pointers.

Supplementary symbols

The elementary symbol is only the anchor. These supplementary marks modify it — where the weld goes, how its face is finished, and where the process is recorded. They are frequently where the inspection characteristics live: a weld-all-around fillet is one requirement, not four.

Symbol	How it’s drawn	What it means (AWS vs ISO)
Weld all around	Small open circle at the junction of the arrow and reference line.	The weld runs all the way around the joint series — one weld type and size, start meets end. Identical in AWS and ISO (ISO Table 3 No. 10).
Field / site weld	Filled flag on a short pole at the arrow/reference junction.	Weld is made at the installation site, not in the shop. Same flag in both standards (ISO No. 12).
Contour — flush / flat	Straight line placed over or under the weld symbol.	Weld face is finished level with the parent surface. Same in both; ISO also allows an ISO 1302 finish symbol alongside.
Contour — convex	Arc bowed away from the weld symbol.	Weld face is crowned. Same in both standards.
Contour — concave	Arc bowed toward the weld symbol.	Weld face is dished for a fatigue-friendly toe. Same in both standards.
Finishing method (C, G, H, M, R, U)	A letter placed against the contour symbol.	How the contour is produced: C chipping, G grinding, H hammering, M machining, R rolling, U unspecified. AWS convention only; ISO instead uses a “blend the toes smoothly” symbol plus tail / WPS notes.
Melt-through	Filled semicircle opposite the weld symbol.	Full penetration with visible root reinforcement. AWS calls it melt-through; ISO calls it specified root reinforcement (No. 6), which in System B doubles as the flanged designator.
Backing	Rectangle opposite the groove symbol.	A backing bar supports the root. AWS puts R inside when the backing is removed after welding; ISO Nos. 7a–7c mark it plain (unspecified), M permanent, or MR removable.
Spacer	Rectangle breaking the reference line.	A spacer strip sits in the joint root. Same in both standards (ISO No. 8).
Consumable insert	Small square opposite or below the weld symbol.	A pre-placed root insert is consumed into the root pass. Both standards (ISO No. 9); AWS states the insert class in the tail.
Staggered intermittent	Two fillet triangles offset above/below the line with length–pitch numbers.	Intermittent welds are staggered side to side. AWS offsets the triangles; ISO System A uses a Z stroke on the reference line with n × l (e). Chain intermittent keeps the symbols aligned in both.
Tail	Open fork at the far end of the reference line.	Carries process, spec, and note references; omitted when unused. AWS uses process letters (A2.4 Annex A); ISO uses numeric process references (ISO 4063, e.g. 135 = MAG).

What else rides on a weld symbol?

The basic symbol is only the anchor. A circle at the bend of the reference line makes the weld all-around; a flag marks a field weld; a tail carries process or spec references (for aerospace work, commonly the welding procedure and class); and finish contour symbols sit over the weld symbol itself. For inspection, those supplementary marks are frequently where the FAIR characteristics live — an all-around fillet is one requirement, not four.