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.

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