set square

noun2

1. •
   (UK) A flat triangular piece of plastic or other material, having corners of precise angles, used in technical drawing

   • Descartes extended this classification by introducing his set-square compass. This is mechanism of sliding set squares that "push" each other... . — 2008
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.