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.

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°

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.

So far, you have learned that angles form when two sides meet up at a common endpoint and are named according to their degree measure and their groupings with other angles. Other important angles to consider include interior and exterior angles. Interior angles occur inside a shape, and exterior angles form outside of a shape.

Alternate interior angles are created when a transverse line passes through a pair of lines. The alternate interior angles are on the inside of the two lines, on alternate sides of the transverse line. If the pair of lines are parallel, the alternate interior angle theorem states that the alternate interior angles are congruent.

**Answers to Practice Problems**

- B
- D
- D
- C
- A

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