set square

set square (plural set squares)

1. (Britain) 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