Try this Drag the orange dot at point P.
Note that the central angle∠AOB is always twice the
inscribed angle∠APB.

The Central Angle Theorem states that the measure of
inscribed angle (∠APB)
is always half the measure of the central angle∠AOB.
As you adjust the points above, convince yourself that this is true.

Exception

This theorem only holds when P is in the
major arc.
If P is in the
minor arc
(that is, between A and B) the two angles have a different relationship.
In this case, the inscribed angle is the
supplement
of half the central angle. As a formula:
In other words, it is 180 minus what it would normally be.