Names of Angles

As the Angle Increases, the Name Changes:

types of angle
Type of AngleDescription
Acute Angleis less than 90°
Right Angleis 90° exactly
Obtuse Angleis greater than 90° but
less than 180°
Straight Angleis 180° exactly
Reflex Angleis greater than 180°
Full Rotationis 360° exactly
Try It Yourself:
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In One Diagram

This diagram might make it easier to remember:
Also: Acute, Obtuse and Reflex are in alphabetical order.  
angle types together
acute
Also: the letter "A" has an acute angle.

Be Careful What You Measure

angle obtuse vs reflex
The smaller angle is an Obtuse Angle,
but the larger angle is a Reflex Angle
So when naming the angles make sure that you know which angle is being asked for!

Positive and Negative Angles

When measuring from a line:
  • positive angle goes counterclockwise (opposite direction that clocks go)
  • negative angle goes clockwise
positive negative angle

Example: −67°

negative 67 degrees

angle 

Parts of an Angle 

The corner point of an angle is called the vertex
And the two straight sides are called arms
The angle is the amount of turn between each arm.

How to Label Angles

angles
There are two main ways to label angles:
1. 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)
2. or by the three letters on the shape that define the angle, with the middle letter being where the angle actually is (its vertex).
Example angle "a" is "BAC", and angle "θ" is "BCD"

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