An angle is made of two rays that share an endpoint. That common endpoint is called the vertex of the angle. The two rays are called the sides. The space between the two sides is called the interior of the angle. The angle in the diagram above can be named <ABC, <CBA, <B, or <1. It *cannot be named*: <CAB, <BAC, <BCA, or <ACB, because those orders of points don’t outline the actual angle pictured (the vertex B isn’t named in the middle of the order like it is supposed to be).

Note: Using the version of naming <B (only using the vertex) only works when only one angle is using that point as the vertex. If more than one angle are using a vertex, you must use the three point method.

The size of the opening of an angle is described using degrees.

Angles can be classified as:

Acute- measure is less than 90º

Right- measure is equal to 90º

Obtuse- measure is greater than 90º, and less than 180º

Straight- measure is equal to 180º

**Types of Angle Pairs**

Complementary- two or more angles whose measures add up to 90º

Supplementary- two or more angles whose measures add up to 180º

Adjacent- two angles that share a common vertex, a common side, and whose interiors don’t overlap.

Linear Pair- two adjacent angles whose non-shared sides form opposite rays.

Vertical Angles- the non-adjacent angles formed by two intersecting lines.