Angles

An angle measures the amount of turn

Names of Angles

As the Angle Increases, the Name Changes:

Type of Angle Description
Acute Angle an angle that is less than 90°
Right Angle an angle that is 90° exactly
Obtuse Angle an angle that is greater than 90° but
less than 180°
Straight Angle an angle that is 180° exactly
Reflex Angle an angle that is greater than 180°

types of angle

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.  

Be Careful What You Measure

Obtuse Angle Reflex Angle
This is an Obtuse Angle
And this is a Reflex Angle
 

But the lines are the same ... 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:

  • a positive angle goes counterclockwise (opposite direction that clocks go)
  • a negative angle goes clockwise

Example: −67°


 

 

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

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"