T-square

 t-square It may also guide a set square to draw vertical or diagonal lines. Its name comes from its resemblance to the letter T. 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.