# Angles

**An angle measures the amount of turn**

## Names of Angles

### As the Angle Increases, the Name Changes:

Type of Angle | Description |
---|---|

Acute Angle | is less than 90° |

Right Angle | is 90° exactly |

Obtuse Angle | is greater than 90° but less than 180° |

Straight Angle | is 180° exactly |

Reflex Angle | is greater than 180° |

Full Rotation | is 360° exactly |

Try It Yourself:

## In One DiagramThis diagram might make it easier to remember: Also: |

Also: the letter "A" has an acute angle.

## 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 |

## 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**"