Table of Contents
Definition
An angle measures the amount of turn
Names of Angles
As the Angle Increases, the Name Changes:
Type of Angle | Description | Illustration |
Acute Angle | is less than 90° | ![]() |
Right Angle | is 90° exactly | ![]() |
Obtuse Angle | is greater than 90° butless than 180° | ![]() |
Straight Angle | is 180° exactly | ![]() |
Reflex Angle | is greater than 180° | ![]() |
Full Rotation | is 360° exactly | ![]() |
Positive and Negative Angles
When measuring from a line:
a positive angle goes counterclockwise (opposite direction that clocks go) | ![]() |
a negative angle goes clockwise | ![]() |
Elements of an Angle
- The corner point of an angle is called the vertex
- And the 2 straight sides are called arms
- The angle is the amount of turn between each arm.
Labeling an Angle
There are two main ways to label angles:
- By the three letters on the shape that define the angle, with the middle letter being where the angle actually is (its vertex). Example BAC and BCD
- Give the angle a name, usually a lower-case letter like a or b, or sometimes a Greek letter like α (alpha) or θ (theta)