AWS A2.4 (American Welding Society) and ISO 2553 are the two principal weld symbol standards. The key structural difference is that pre-2013 ISO drawings used a dashed identification line to indicate the other side, while AWS A2.4 always uses symbol position above or below the reference line. The 2013 revision of ISO 2553 eliminated the dashed line and aligned closely with AWS practice. Additional differences: ISO uses process numbers (ISO 4063) in the tail while AWS uses letter codes; ISO uses “a/z” prefixes to distinguish throat from leg size, while AWS uses leg size without prefix. The all-around circle and field weld flag are the same in both standards, except the AWS flag is open while the ISO flag is filled.

In AWS A2.4, the number to the left of a fillet weld symbol is the weld leg size in the drawing units (inches or millimetres). For an equal-leg fillet, only one number is given. For an unequal-leg fillet, both legs are shown in parentheses, e.g. (6)(10). The effective throat is not explicitly stated for fillet welds in AWS — it is calculated as 0.707 × the leg size and used in design calculations. In ISO 2553, the prefix “z” before the number explicitly identifies it as a leg dimension (e.g. z8); the prefix “a” or “s” identifies the design throat (e.g. a6).

A circle at the junction of the arrow and reference line means the weld is to be made all around the joint — the all-around symbol. This specifies a continuous weld on every accessible side of the feature shown. It is used for full perimeter fillet welds around a pad or boss, seal welds around a socket, or any joint where weld continuity on all sides is structurally or leak-tightness required. The symbol is the same in both AWS A2.4 and ISO 2553. It does not specify the number of sides — it means all sides visible and accessible in the configuration shown on the drawing.

A flag (small pennant shape) at the elbow of the reference line indicates a field weld — a weld to be made at the installation site rather than in the fabrication shop. This is important for work planning, equipment mobilisation, WPS site qualification, and weather/access considerations. In AWS A2.4 the flag is an open (unfilled) pennant; in ISO 2553 the flag is filled (solid). Both can appear together with the all-around circle when a weld is both all-around and a field weld.

In AWS A2.4, a groove weld with complete joint penetration (CJP) is specified by using the appropriate groove symbol (V, bevel, J, U, etc.) with no number to the left of the symbol when prequalified joint details are used. The notation “CJP” may be added in the tail for clarity. For partial joint penetration (PJP) groove welds, the effective throat is given in parentheses to the left of the symbol. In ISO 2553, the designation “ss nb” (single side, no backing) or “ss mb” (single side, with melt-back) in the tail confirms full penetration intent.

The effective throat of a standard equal-leg fillet weld with leg size w is t_e = 0.707 × w (the perpendicular distance from the root to the weld face). This is the dimension used in structural design calculations for weld load capacity. For a 10 mm leg fillet, the effective throat is 7.07 mm. AWS D1.1 and Eurocode 3 EN 1993-1-8 both use the effective throat (design throat) for weld capacity calculations. For deep-penetrating GMAW fillet welds that achieve root penetration beyond the theoretical root, an enhanced effective throat may be used if qualified by testing, per AWS D1.1 Clause 3.6.

Intermittent fillet weld symbols include a length-pitch notation to the right of the weld symbol in the format L-P: L is the length of each weld increment and P is the centre-to-centre pitch. For example, “50-150” means 50 mm long welds at 150 mm centre-to-centre spacing. Chain intermittent welds have the symbols on both sides of the reference line at the same positions. Staggered intermittent welds have the symbols offset — the symbols on opposite sides of the line are not aligned, creating an alternating pattern that distributes stress concentration and improves fatigue performance.

The tail in AWS A2.4 contains supplementary information that cannot be shown by standard symbols: welding process designation (SMAW, GMAW, GTAW etc.); WPS reference number; filler metal classification (e.g. E7018, ER70S-6); PWHT requirements; NDE requirements (e.g. RT, UT, MT); acceptance criteria references; or notes referring to detail drawings. If no supplementary information is needed, the tail is omitted entirely. ISO 2553 uses the same convention with ISO 4063 process numbers and metric-prefixed dimensions in the tail.

In AWS A2.4, the melt-through symbol is a filled (solid black) circle placed on the reference line on the opposite side from the groove weld symbol. It indicates that complete root fusion and penetration is required from one side, producing a visible root reinforcement bead on the back face without a separate backing weld. This is used for single-sided groove welds where back-side access for back-gouging is unavailable, such as small-diameter pipe root passes, closed box sections, or orbital GTAW pipe welds. The required root reinforcement height may be specified with a dimension alongside the melt-through symbol.