Inscribed angle

In geometry, an inscribed angle is the angle formed in the interior of a circle when two chords intersect on the circle. It can also be defined as the angle subtended at a point on the circle by two given points on the circle.

The inscribed angle theorem, sometimes called the central angle theorem, relates the measure of an inscribed angle to that of the central angle intercepting the same arc. Its oldest appearance is in Propositions 20–21 in Book 3 of Euclid's Elements.