"right-angle" is referenced in the pages listed below.

Perpendicular definition: Perpendicular simply means 'at right angles'. A line is perpendicular to another if they meet at 90 degrees.
On this page we show how to construct (draw) a 90 degree angle with compass and straightedge or ruler. There are various ways to do this, but in this construction we use a property of Thales Theorem. We create a circle where the vertex of the desired right angle is a point on a circle. Thales Theorem says that any diameter of a circle subtends a right angle to any point on the circle. A Euclidean construction.