Angles are geometric figures formed by two rays having the same endpoint. The two rays are called the sides of the angle, and their common endpoint the vertex of the angle.

An angle is often denoted by the symbol Ð , followed by a number, a letter or three letters. If an angle is named using letters, the middle letter denotes the vertex of the angle, while the other letters refer to points on the sides of the angle; if there is only one letter, it then refers to the vertex. The above angle can be referred to as
Ð 1, Ð B or Ð ABC.

As the vertex B is the vertex of three different angles, the notation Ð B would be ambiguous. The three-letter rule for naming angles would be appropriate here. Thus, while Ð B is inexact in naming the above angles, ÐABC , ÐABD,
ÐCBD each refer to a unique angle.