Thales's theorem

A graph demonstrating Thales's Theorem.

Thales's theorem is a theorem (rule) in geometry. It says that if three points, $P$, $Q$, and $R$ are on a circle where $PR$ is a diameter (a straight line from one end of the circle to another, through the middle), the angle $PQR$ is a right angle of 90 degrees. This is a special case of the inscribed angle theorem.