Angles

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°

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

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!

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.

Labelling Angles

There are two main ways to label angles: 1. by giving 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"