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.
The above animation is available as a printable step-by-step instruction sheet, which can be used for making handouts or when a computer is not available.
This construction works by using Thales theorem. It creates a circle where the apex of the desired right angle is a point on a circle.
|1||The line segment AB is a diameter of the circle center D||AB is a straight line through the center.|
|2||Angle ACB has a measure of 90°.||The diameter of a circle always subtends an angle of 90° to any point (C) on the circle. See Thales theorem.|