OBJET D'ART: T-SQUARE

A T-square is a technical drawing instrument used by draftsmen and designers primarily as a guide for drawing horizontal lines on a drafting table. The instrument is named after its resemblance to the letter T, with a long shaft called the “blade” and a short shaft called the “stock” or “head.” T-squares are available in a range of sizes, with common lengths ranging from 18-42 inches.

In addition to drawing horizontal lines, a T-square can be used with a set square to draw vertical or diagonal lines. 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.

Source: Wikipedia