A weld symbol packs the whole weld spec — type, size, length, location — into one compact mark on the drawing. The trick to reading it is knowing that the reference line and the arrow carry the meaning.
Arrow side vs other side
The arrow points to the joint. A weld symbol drawn below the reference line means "weld on the arrow side" (the side the arrow touches); drawn above the line means the other side. Symbols on both sides mean weld both. This is the single most important — and most misread — convention.
What the symbol shape tells you
A triangle is a fillet weld (the everyday weld joining two surfaces at an angle). Different shapes denote groove welds (square, V, bevel, U, J) used for full-strength butt joints. Size the fillet with the Weld tool and keep the strength formulas on the welding cheat sheet.
The numbers
The number to the left of the symbol is the weld size (leg length for a fillet). A number to the right is the weld length, and a dimension in parentheses is the pitch for intermittent welds. A circle at the bend in the arrow means "weld all around"; a flag means "weld in the field," not the shop.
A weld symbol has the triangle drawn below the reference line. Which side of the joint does the weld go on?
Below the line means arrow side. It is arbitrary, it is the opposite of what most people guess, and getting it wrong puts the weld on the wrong face of the joint — which on a bracket that is only accessible from one side means scrapping the assembly. The mnemonic that sticks: the symbol below the line is nearest the arrow, so it belongs to the arrow side. Symbols on both sides of the line mean weld both sides. (This is the ISO and AWS convention; older drawings may differ, which is exactly why the drawing should state the standard.)
Try it — build the symbol, see the joint
Why it matters
Weld toes are stress raisers, so an over-specified weld isn't free — it adds heat, distortion and cost — while an under-specified one fails. Call out exactly the size you need (per fatigue if the joint is cyclically loaded), and follow AWS D1.1 (structural) or D17.1 (aerospace).