A weld symbol below the reference line and the same symbol above it call for completely different welds. AWS A2.4 basics: reference line, arrow side vs. other side, common weld type symbols, and how this differs from GD&T.
A weld symbol below the reference line and the same symbol above it mean
two completely different welds on two different sides of the joint. That
single arrow-side/other-side convention is where most welding symbol
misreads actually happen — not in the exotic supplementary symbols, but
in the basic structure everyone assumes they already understand.
Every AWS A2.4 welding symbol is built from the same three parts: a
horizontal reference line, an arrow (a leader line pointing to the actual
joint), and an optional tail (used for a welding process, specification,
or reference note — omitted when there’s nothing to add). The weld type
symbol itself sits on the reference line, and where it sits relative to
that line is the part worth slowing down for.
A symbol placed below the reference line calls for
the weld on the arrow side of the joint — the side the
arrow physically points to. A symbol placed above the
line calls for the weld on the other side — the side
opposite the arrow. Symbols on both sides mean weld both sides of the
joint. This is an AWS A2.4 convention specifically — if you’re reading a
drawing built to ISO 2553 instead, the convention for arrow side vs.
other side works differently, so check which standard governs the print
before assuming.
| Mistake | What actually happens |
|---|---|
| Welding the wrong side | Confusing arrow-side (below the line) with other-side (above the line) puts the weld on the wrong face of the joint entirely |
| Missing the tail note | An empty tail is normal, but skipping a filled tail means missing a required process or spec reference |
| Assuming symmetric welds | A symbol on only one side of the line means only that side gets welded — nothing on the other side unless it’s called out too |
| Misreading a weld-all-around vs. field weld | A circle at the arrow/reference-line junction means weld all around the joint; a flag at the same spot means the weld happens on-site, not in the shop — confusing the two changes both scope and logistics |
| Weld type | What it looks like | Typical joint |
|---|---|---|
| Fillet | Right triangle | T-joints, lap joints, corner joints |
| Groove (V, bevel, U, J, square) | Varies by groove shape | Butt joints, full-penetration joints |
| Plug / slot | Rectangle | Overlapping plates joined through a hole or slot |
| Spot / seam | Circle (spot) or oval (seam) | Resistance-welded sheet metal joints |
Dimensions follow their own convention too: weld size is given to the
left of the symbol, and length or pitch (for intermittent welds) is given
to the right. A contour symbol — flush, convex, or concave — can sit
above the weld symbol to specify the finished weld’s profile, sometimes
paired with a finish-method letter (G for grind, M for machine, C for
chip, H for hammer, R for roll) showing how that contour gets achieved.
Picture a fillet weld symbol on the reference line, positioned below
it, with “1/4” to the left of the triangle and a weld-all-around circle
at the junction of the arrow and reference line. Read left to right: it’s
a fillet weld (the triangle), a quarter-inch leg size (the “1/4”), placed
on the arrow side (below the line), and it goes all the way around the
joint (the circle) rather than stopping partway. Swap that circle for a
field-weld flag instead, and the same symbol now says the identical weld
gets made after the part ships to the job site, not on the shop floor —
a completely different logistics and scheduling situation from the same
basic symbol shape.
That’s the value of learning the structure instead of memorizing
individual symbols in isolation: once you know reference line, arrow
side/other side, and where dimensions and supplementary symbols attach,
you can read a combination you’ve never seen before instead of needing a
chart for every possible variation.
Welding symbols (AWS A2.4) and GD&T (ASME Y14.5) are two entirely
separate standards answering two different questions. AWS A2.4 controls
how a joint gets welded — type, size, process, and location.
ASME Y14.5 controls how much the resulting part’s geometry is allowed
to vary — form, orientation, and location tolerances. Both can appear
on the same drawing, but neither substitutes for the other, and knowing
one doesn’t mean you automatically know the other. If you want the GD&T
side of reading a drawing, see our related article on
GD&T fundamentals.
Welding symbol reading isn’t a standalone course in SCMEP’s current
catalog — it’s the kind of print-reading skill that feeds directly into
our Inspection learning path, right
alongside GD&T literacy, for verifying what actually got built against
what the drawing specified. As a
NIST Manufacturing Extension Partnership affiliate
serving South Carolina manufacturers since 1989, our focus is
connecting symbol literacy to real inspection and quality workflows, not
just chart memorization.
If your team needs to build stronger blueprint-reading skills across
welding and inspection, you can
browse the Quality and Inspection training
catalog or email the training team.
Welding symbols are one piece of a larger drawing. See our related
guide on how to read a
blueprint for the alphabet of lines, view types, and revision history
that surround those symbols.
A symbol below the reference line calls for the weld on the arrow side — the side the arrow leader line physically points to. A symbol above the line calls for the weld on the other side, opposite the arrow.
A circle at the junction of the arrow and reference line means weld continuously all the way around the joint. A flag or pennant at the same spot means the weld is made on-site (in the field) rather than in the shop. Confusing the two changes both the weld’s scope and where it happens.
No. Welding symbols follow AWS A2.4 and control how a joint is welded — type, size, process, location. GD&T follows ASME Y14.5 and controls how much a part’s finished geometry is allowed to vary. They’re separate standards that can both appear on the same drawing.
Weld size is given to the left of the weld symbol. Length or pitch, for intermittent welds, is given to the right. A contour symbol above the weld symbol can specify the finished profile, sometimes with a finish-method letter showing how it’s achieved.
