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Types of Angles

An angle is formed when two rays share the same endpoint, which is called the vertex of the angle. In geometric notation, the vertex is the middle letter in the name of the angle. Each angle has a degree value (°), which measures how far the ray rotates from the vertex. The degree value determines its angle type. Groupings of angles are labeled by the sum of their angles or how they are formed.

5 Basic Angle Types

You should be familiar with these five basic angle types:

Acute Angle: Less than 90°
Right Angle: Exactly 90°
Obtuse Angle: Between 90° and 180°
Straight Angle: Exactly 180°
Reflex Angle: Between 180° and 360°

Angle Groupings

As you review angle groupings, use these tricks to help you remember the difference between complementary and supplementary angles.

Since complementary angles sum to 90° and form a right angle, remember that paying someone a compliment is the right thing to do.
Because supplementary angles sum to 180° to make a straight line, remember to take your vitamin supplements to keep your bones straight and strong.

All of the angles you just saw are interior angles, which occur inside a shape. Later, you will learn about exterior angles, which form outside of a shape. For now, focus on the various ways of categorizing these angles.