set square

set square (plural set squares)

1. (UK) A flat triangular piece of plastic or other material, having corners of precise angles, used in technical drawing
   • 2008 January, Michel Serfati, “A Note on the Geometry and Descartes's Mathematical Work”, in Notices of the American Mathematical Society, volume 55, number 1, pages 50–51:

     Descartes extended this classification by introducing his set-square compass. This is mechanism of sliding set squares that "push" each other... .

2. A right angle tool used to determine if two surfaces join at a 90-degree angle, composed of two arms, usually made from metal.

• (flat triangular piece of plastic): triangle

• sequestra