A T-square is a technical drawing instrument used by draftsmen primarily as a guide for drawing horizontal lines on a drafting table. It may also guide a set square to draw vertical or diagonal lines. Its name comes from its resemblance to the letter T. T-squares come in varying sizes, common lengths being 18 inches (460 mm), 24 inches (610 mm), 30 inches (760 mm), 36 inches (910 mm) and 42 inches (1,100 mm).
T-squares are also used to measure and cut drywall. Drywall T-squares are typically made of aluminium and have a 48-inch (1,200 mm) long tongue.
Higher-end table saws are often equipped with T-square fences. These table saw fences are attached to a rail only on the front side of the table unlike traditional fences which clamp to both the front and the rear of the table.
The top of the T-square hangs off of the drafting table and is called the head; the blade is the part that stays on the drafting table. The T-square usually has a transparent edge made of plastic which should be free of nicks and cracks in order to provide smooth, straight lines.