Carpenter Squares

A carpenter square (also called a framing square) is a flat, L-shaped steel or aluminum square with a long body (typically 24") and a shorter tongue (typically 16"), stamped with a set of scales and tables used for layout work well beyond simply checking a 90° angle. It's one of the oldest layout tools still in common use, and the printed tables on a quality one are effectively a slide-rule for stair and roof-framing math.

What the tables are actually for

Framing squares are commonly stamped with rafter tables (for calculating rafter lengths at various roof pitches) and, on many models, a set of graduated scales usable for stair stringer layout — marking consistent rise and run increments directly onto a stringer board without doing the trigonometry by hand each time. This is squarely construction-trade territory, but it's the reason the tool has "framing" in one of its common names.

What it's good for in a woodshop

• Large layout work — checking or marking squareness across a wider span than a smaller square can reach, useful for cabinet carcasses, large panels, or checking that a big glue-up came out square.
• General reference straightedge — the long body doubles as a reasonably reliable straightedge for marking long lines, though a dedicated straightedge is more accurate for critical work.

Where it's not the right tool

Fine joinery layout (too large and not precise enough — see Machinist Squares or a smaller try square instead), and quick single-line crosscut marking, where a Speed Square is faster to use one-handed.

Checking accuracy

Same test as any square: mark a line against a known-straight edge, flip the square, mark again from the same reference point. A visible gap between the two lines reveals the error, doubled — worth doing on a new square, and worth repeating occasionally on one that gets knocked around a job site or shop floor regularly.