An angle is the union of two rays with a common endpoint. The common endpoint of the rays is called the vertex of the angle, and the rays themselves are called the sides of the angle. A ray drawn from the vertex of the angle, such that the angle formed by this ray and one of the sides is congruent to the angle formed by this ray and the other side, is called the angle bisector.
There are many notations for angles. The most common form is , read "angle ABC", where are points on the sides of the angle and is the vertex of the angle. Note that the same angle can be denoted many different ways by choosing different points along the side of the angle. If there is no ambiguity, this notation can be shortened to simply .
Congruent angles have the same angle measure.
An acute angle has a measure greater than zero but less than that of a right angle, i.e. is acute.
An obtuse angle has a measure greater than that of a right angle but less than that of a straight angle, i.e. is obtuse.