Two or more reference lines on a welding symbol indicate a sequence of operations. Per A2.4 §6.7.1, the line nearest the arrow is the first operation. Each additional line specifies the next step — for example, weld then backgouge then back weld per D1.1 §7.4.
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Reading Order — Arrow to Outward
3-line sequence: weld → BG → back weld
2-line: groove weld → RT inspect
A2.4 §6.7.1
How to Read the Sequence
The rule is simple: closest to the arrow = first operation. Each additional reference line specifies the next step.
Common 3-line sequences: 1. V-groove weld (1st line) → backgouge (2nd line, BG in tail) → back weld (3rd line) 2. Groove weld (1st line) → RT inspection (2nd line) → flush grind (3rd line) 3. Backing weld (1st line, closer to arrow) → groove weld (2nd line) — note the backing comes before the groove
Per §6.7.2, the tail of additional reference lines may carry supplementary data such as process specifications or standard references.
Per §6.7.3, supplementary symbols (field weld flag, weld-all-around circle) are placed on the reference line for each operation to which they apply — not just the first line.
A2.4 §7.7.2–7.7.3
Back Weld vs Backing Weld on Multiple Lines
The distinction between back welds and backing welds is clearly expressed through reference line order:
Back weld = applied after the groove weld → placed on a reference line subsequent to (farther from the arrow than) the groove weld line.
Backing weld = applied before the groove weld → placed on a reference line prior to (closer to the arrow than) the groove weld line.
When a single reference line is used instead, the distinction is made in the tail: “BACK WELD” or “BACKING WELD” is written in the tail of the welding symbol.
A2.4 §6.8 · Figure 7.21
Subreference Lines — Combination Joints
Subreference lines are different from multiple reference lines. Per §6.8, they are used for joints that transition from one weld type to another around a continuous path (pipe, round bar). Individual weld symbols are placed on their own subreference lines branching from a single reference line.
Key difference: Multiple reference lines (§6.7) = sequence (order matters). Subreference lines (§6.8) = no implied sequence (welding order is undetermined). Do not confuse the two — a subreference line does not mean “do this second.”
CWI Exam Tip: The most commonly tested concept about multiple reference lines is the reading order: nearest to arrow = first operation. The second most tested is back weld (after, subsequent line) vs backing weld (before, prior line). A question might show a 2-line symbol and ask which operation comes first.
Common Questions
Multiple Reference Lines FAQ
What do multiple reference lines mean on a welding symbol?
Multiple reference lines on a welding symbol indicate a sequence of operations per AWS A2.4:2020 Section 6.7.1. The reference line nearest the arrow specifies the first operation. Each additional reference line, moving away from the arrow, specifies a subsequent operation. For example, a symbol with two reference lines might show a groove weld on the first line (weld first) and a back weld on the second line (apply after the groove weld). This is commonly used when backgouging and back welding are required after the initial groove weld, or when NDE inspection must follow a specific weld. A third reference line specifies the third operation in the sequence. The arrow always points to the joint, and the sequence reads from the closest reference line outward.
Which reference line is the first operation?
The reference line nearest the arrow is always the first operation per AWS A2.4 Section 6.7.1. Additional reference lines specify subsequent operations in order, moving away from the arrow. This convention is consistent and unambiguous: the arrow points to the joint, and the closest reference line tells you what to do first. For example, in a three-reference-line symbol: the first line (closest to arrow) might specify a single-V groove weld, the second line might specify backgouging, and the third line might specify a back weld. The welder reads the symbol from the arrow outward and performs operations in that sequence.
What are subreference lines?
Subreference lines are used for combination weld joints where the joint transitions from one weld type to another around a continuous path, such as pipe or round bar. Per AWS A2.4 Section 6.8, individual weld symbols are placed on their own subreference lines connected to a single reference line. Unlike multiple reference lines under Section 6.7, subreference lines do not imply a sequence of operations. The welding order with respect to subreference lines is undetermined. The Figure 7.21 example shows a cylindrical joint with different groove types around its circumference, each specified on its own subreference line branching from the main reference line.
How are back welds shown with multiple reference lines?
Per AWS A2.4 Sections 7.7.2 and 7.7.3, when using multiple reference lines, the back weld symbol is placed on a reference line subsequent to (farther from the arrow than) the groove weld reference line. This is because a back weld is applied after the groove weld is completed. Alternatively, when a single reference line is used, the back weld is specified in the tail. A backing weld, which is applied before the groove weld, is placed on a reference line prior to (closer to the arrow than) the groove weld. This distinction between back weld (after) and backing weld (before) is one of the most commonly tested concepts on the CWI exam.