A T-square is a technical drawinginstrument used by draftsmen primarily as a guide for drawing horizontal lines on a drafting table. It may also guide a triangle to draw vertical or diagonal lines. Its name comes from the general shape of the instrument where the horizontal member of the T slides on the side of the drafting table. T-squares come in varying sizes, common lengths being 18", 24", 30", 36" and 42".
T-squares are also used to measure and cut drywall. Drywall T-squares are typically made of aluminium and have a 48-inch (1,220 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.
T-squares have been used to cut drawings created by draftsmen since the inception of the instrument. This is inadvisable: One might cut the T-square in the process, damaging the straight edge and rendering the T-square useless. However, extremely advisable when using a metal T-square.
