Every key term used in welding symbol notation, defined in plain English with references to the relevant ISO 2553 and AWS A2.4 clauses. Use this as a quick lookup when reading drawings or studying for certification.
The line connecting the reference line to the joint on the drawing. It points to the exact location where the weld is to be made. The arrow line may be straight or have a bend (a bent arrow indicates the member that receives the weld preparation in an asymmetric joint, such as a bevel or J-groove).
ISO 2553 Clause 4.2; AWS A2.4 Section 3
The side of the joint to which the arrow line points. In ISO System A, the arrow side is indicated by placing the weld symbol on the solid reference line. In AWS A2.4 and ISO System B, the arrow side is indicated by placing the weld symbol below the reference line.
ISO 2553 Clause 4.3; AWS A2.4 Section 3
A weld run deposited on the back (root side) of a joint after the main weld has been completed, typically to ensure full penetration. Sometimes called a back weld or backing run. Indicated by the backing/back weld supplementary symbol on the opposite side of the reference line from the main weld symbol.
ISO 2553 Clause 5.4.5
A weld deposited on the root side of a joint before the main weld is applied, providing a surface for the root pass to fuse into. Distinguished from a back run in that it is made before the main weld, not after. Symbolised by a half-circle on the opposite side of the reference line.
ISO 2553 Clause 5.4.5; AWS A2.4 Section 3
See Elementary symbol. In AWS terminology, 'basic weld symbol' refers to the symbol (shape) indicating the type of weld, placed on or against the reference line.
AWS A2.4 Section 2
A weld in a joint between two members lying approximately in the same plane. Also called a groove weld (AWS terminology). Types include square butt, single-V, double-V, single-U, double-U, single-bevel, double-bevel, single-J, and double-J.
ISO 2553 Table 1; AWS A2.4 Table 1
An intermittent weld on both sides of a joint where the weld increments are positioned directly opposite each other (not staggered). The symbol shows the intermittent notation on both sides of the reference line, aligned.
ISO 2553 Clause 6.2.4; AWS A2.4 Section 3
Additional data placed in the tail or alongside the welding symbol, such as the welding process number, quality level, filler material specification, or non-destructive testing requirements. Separated by forward slashes when multiple items are present.
ISO 2553 Clause 7
A pre-placed filler metal ring or strip positioned in the root of a joint before welding. It melts and becomes part of the weld. Used primarily in pipe and tube welding to ensure consistent root penetration. Shown as a square symbol on the opposite side of the reference line from the groove weld symbol.
AWS A2.4 Section 3
The nominal throat dimension of a fillet weld that includes penetration beyond the root of the joint, denoted by the prefix 's' in ISO notation. Written as, for example, s8 to indicate 8 mm of nominal plus deep penetration throat. Only used where the welding process reliably produces deeper-than-standard fusion.
ISO 2553 Clause 6.2.2
A weld made on the edges of two or more members, typically where the edges are parallel or nearly parallel. Used for sheet metal joints and flange welds. The elementary symbol is a square shape sitting on the reference line.
ISO 2553 Table 1; AWS A2.4 Table 1
The geometric shape placed on the reference line that indicates the type of weld to be made. Also called the basic weld symbol (AWS) or basic symbol. Each weld type has a distinct shape: triangle for fillet, V for V-butt, half-circle for J-butt, and so on.
ISO 2553 Clause 5.2; AWS A2.4 Section 2
A weld that is to be made at the construction site (in the field) rather than in the workshop or factory. Indicated by a flag symbol at the junction of the arrow line and reference line. The flag points away from the arrow.
ISO 2553 Clause 5.4.3; AWS A2.4 Section 3
A weld of approximately triangular cross-section joining two surfaces at an angle to each other, typically 90 degrees. The most common weld type in structural steelwork and general fabrication. Dimensioned by throat thickness (a) or leg length (z) in ISO, or by leg size alone in AWS.
ISO 2553 Table 1; AWS A2.4 Table 1
A weld in a groove formed between a curved surface (such as a tube or bar) and a flat surface. The elementary symbol is a flared bevel shape. Often encountered in tubular steel connections.
ISO 2553 Table 1; AWS A2.4 Table 1
A weld in a groove formed between two curved surfaces, such as two round bars or tubes placed side by side. The elementary symbol is two mirrored flare-bevel shapes forming a V.
ISO 2553 Table 1; AWS A2.4 Table 1
A contour supplementary symbol indicating that the weld face should be finished smooth and level with the parent material surface. Shown as a straight horizontal line above the weld symbol. May require post-weld machining or grinding.
ISO 2553 Clause 5.4.1; AWS A2.4 Section 3
A spot weld made by an arc welding process (as opposed to resistance welding). Produces a localised fusion joint between overlapping members. Indicated by a circle symbol. The 'fusion' prefix distinguishes it from a resistance spot weld.
ISO 2553 Table 2; AWS A2.4 Table 1
The included angle between the prepared surfaces of a groove (butt) joint, measured before welding. In ISO, the angle is placed inside the weld symbol. In AWS, it is placed outside the symbol. Typical values are 60 degrees for a V-groove and 45 degrees for a bevel.
ISO 2553 Clause 6.3; AWS A2.4 Section 3
A weld that is not continuous along the full length of the joint but is applied in a series of increments with gaps between them. Specified by length and pitch: l(e), where l is the increment length and e is the centre-to-centre pitch. Can be chain or staggered.
ISO 2553 Clause 6.2.4; AWS A2.4 Section 3
The distance from the root of a fillet weld to the toe, measured along the surface of one of the joined members. For an equal-leg fillet, both legs are the same length. In ISO notation, prefixed with 'z' (e.g. z6 = 6 mm leg). AWS uses the bare number and assumes leg length.
ISO 2553 Clause 6.2.2; AWS A2.4 Section 3
The minimum distance from the root of a fillet weld to the face, measured perpendicular to the face. Also called the design throat. In ISO, prefixed with 'a' (e.g. a5 = 5 mm throat). For an equal-leg fillet, a = z x 0.707. AWS does not use throat directly in the symbol but references effective throat in calculations.
ISO 2553 Clause 6.2.2
The specified length of a weld, placed to the right of the elementary symbol. For a continuous weld, this is the total length. For an intermittent weld, it is expressed as the length of each increment. If no length is given, the weld is assumed to extend the full length of the joint.
ISO 2553 Clause 6.2.3; AWS A2.4 Section 3
See Staggered intermittent weld. An offset (staggered) arrangement places weld increments on opposite sides of a joint in alternating positions rather than directly opposite each other.
ISO 2553 Clause 6.2.4
The side of the joint opposite the arrow side (i.e. the face of the joint that the arrow does not touch). In ISO System A, indicated by placing the weld symbol on the dashed reference line. In AWS and ISO System B, indicated by placing the symbol above the reference line.
ISO 2553 Clause 4.3; AWS A2.4 Section 3
A weld deposited on an unjointed surface for the purpose of building up material or providing a corrosion-resistant, wear-resistant, or hard-facing layer. Also called surfacing. The symbol is a series of parallel horizontal lines on the reference line.
ISO 2553 Table 2; AWS A2.4 Table 1
The distance from the original surface of the base metal to the furthest point of weld fusion. For groove welds, this may be partial or full (complete joint penetration). Indicated by a dimension to the left of the groove weld symbol, inside parentheses in AWS notation.
ISO 2553 Clause 6.3; AWS A2.4 Section 3
A weld made in a circular hole in one member of a lap joint, fusing that member to the other. The hole may be completely or partially filled. The symbol is a filled rectangle. Dimensions specify the hole diameter and, optionally, pitch and number of plugs.
ISO 2553 Table 2; AWS A2.4 Table 1
A resistance weld made at predetermined points by means of projections (raised bumps) on one of the mating surfaces. The projections localise the welding current. Related to spot welding but uses prepared projections rather than flat sheet overlap.
ISO 2553 Table 2; AWS A2.4 Table 1
The horizontal line at the centre of every welding symbol. All weld type symbols, dimensions, and supplementary symbols are positioned relative to it. ISO System A uses a dual reference line (solid + dashed). AWS and ISO System B use a single solid line.
ISO 2553 Clause 4.1; AWS A2.4 Section 3
A continuous weld formed between overlapping members by passing them between rotating wheel electrodes that apply pressure and current. Creates a gas-tight or liquid-tight seam. The symbol is a circle with two parallel horizontal lines through it.
ISO 2553 Table 2; AWS A2.4 Table 1
A weld made between overlapping members by clamping them between pointed electrodes and passing a high current for a brief time. The fused zone forms a circular nugget. The symbol is a circle. Dimensions specify nugget diameter and pitch.
ISO 2553 Table 2; AWS A2.4 Table 1
The separation between the members at the root of the joint before welding. Also called root opening. Specified to allow adequate penetration. Placed inside the weld symbol in ISO, or at the inside of the groove symbol in AWS.
ISO 2553 Clause 6.3; AWS A2.4 Section 3
Excess weld metal protruding from the root side of a weld beyond the plane of the back surface of the base metal. A small amount is generally acceptable; excessive reinforcement may require grinding.
ISO 5817; AWS D1.1
A continuous linear weld, either resistance seam (made by wheel electrodes) or fusion seam (made by arc or beam). Produces a gas-tight joint along the length of overlapping sheets.
ISO 2553 Table 2; AWS A2.4 Table 1
A metal strip placed in the root of a groove joint before welding. It remains as part of the completed joint and assists with root gap maintenance. Indicated by a rectangle symbol in the root of the groove weld symbol on the reference line.
AWS A2.4 Section 3
A localised weld made at a single point, either by resistance (electrodes pressing overlapping sheets) or fusion (arc creating a nugget). See Resistance spot weld and Fusion spot weld for the distinction.
ISO 2553 Table 2; AWS A2.4 Table 1
An intermittent weld applied to both sides of a joint where the weld increments on one side are offset (staggered) relative to those on the other side, rather than being directly opposite. The symbol shows the intermittent notation on both sides of the reference line with a Z-like offset indicator.
ISO 2553 Clause 6.2.4; AWS A2.4 Section 3
A weld made through one member into or onto another without a prepared hole. Similar in function to a plug weld but without pre-drilling. Often used in sheet metal work.
ISO 2553 Table 2
A weld that attaches a stud (bolt, pin, or similar fastener) to a workpiece. The stud is held in a gun, an arc is drawn, and the stud is plunged into the molten pool. The elementary symbol is a circle with a line extending from it.
ISO 2553 Table 2; AWS A2.4 Table 1
A symbol added to the elementary (basic) weld symbol to provide additional information about the weld shape, extent, or site conditions. Examples include contour symbols (flush, convex, concave), weld-all-round, field weld, and backing.
ISO 2553 Clause 5.4; AWS A2.4 Section 3
The forked end of the reference line, opposite the arrow. Used to carry complementary information such as the welding process number (ISO 4063), quality level (ISO 5817), and filler material specification. Multiple items are separated by forward slashes. The tail is omitted if no additional information is required.
ISO 2553 Clause 7; AWS A2.4 Section 3
A supplementary symbol (circle) placed at the junction of the arrow line and reference line, indicating that the weld extends continuously around the entire perimeter of the joint. Used for circumferential welds on tubes, pipes, and structural connections.
ISO 2553 Clause 5.4.2; AWS A2.4 Section 3
The complete graphical representation on a drawing that communicates all welding instructions for a joint. Includes the reference line, arrow, elementary (basic) symbol, dimensions, supplementary symbols, and tail. Not to be confused with the elementary symbol (the shape indicating the weld type), which is just one component.
ISO 2553 Clause 4; AWS A2.4 Section 2
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