set square

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A flat triangular piece of plastic or other material, having corners of precise angles, used in technical drawing
• noun a right angle tool used to determine if two surfaces join at a 90-degree angle, composed of two arms, usually made from metal.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a try square with an adjustable sliding head