In measurement, a device consisting of two straightedges set at a right angle. It is used by carpenters and machinists to check the correctness of right angles, as a guide when drawing lines on materials before cutting, or for locating holes. In mechanical drawing or drafting, a T-shaped instrument known as a T square is used to establish a horizontal reference on the drafting board.

This entry comes from Encyclopædia Britannica Concise.
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