OfficeSupplies101 has all of the Drafting Tools and Templates for your creative needs. A T-square is a technical drawing instrument, primarily a guide for drawing horizontal lines on a drafting table. It is most commonly used by draftsmen & artists. A T Square is also used to guide the Drafting Triangle that draws vertical 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. The top of a T square hangs off of an art & drafting table and is called the head, whereas 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 and artists since the inception of the instrument. T Squares are available in 24" or 36" length.