The angle subtended at the centre of the circle is twice the angle subtended at the edge.

The angle at the edge of a circle subtended by a diameter is a right angle.

Opposite angles in a cyclic quadrilateral - where all the vertices are an the edge of the circle - add up to 180 degrees.

An angle between a tangent and a chord through the point of contact is equal to the angle in the alternate segment.

Angles in the same segment of a circle subtended from the same chord are equal.

Angles formed from two points on the circumference are equal to other angles, in the same arc, formed from those two points.