An angle measures the amount of turn

Names of Angles

As the Angle Increases, the Name Changes:

Type of AngleDescriptionIllustration
Acute Angleis less than 90° Acute Angles
Right Angleis 90° exactly       Right Angles
Obtuse Angleis greater than 90° butless than 180°  Obtuse Angles
Straight Angleis 180° exactly Straight Angles
Reflex Angleis greater than 180°  Reflex Angles
Full Rotationis 360° exactly Full Rotation

Positive and Negative Angles

When measuring from a line:

a positive angle goes counterclockwise
(opposite direction that clocks go)
 Positive Angles 
a negative angle goes clockwise Negative Angles

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)

Learn More

Acute Angles

Reflex Angle

Obtuse Angles

Adjacent Angles

Geometry Index