Angle measurement — introduction

Angle measurement — introduction

Angle measurement

An angle is the amount of turn between two lines that meet at a point. You see angles every time you open a door, turn a key, or look at the hands of a clock.

The pieces of an angle

WordWhat it means
vertexthe point where the two lines meet
ray (or arm)each of the two lines coming out of the vertex
degree (°)the unit we use to measure how much an angle turns

A small turn is a small angle. A big turn is a big angle. A full spin all the way back to the start is 360°.

An angle: two rays meeting at a vertex, with the size of the turn highlighted

Why 360 degrees?

People have used 360° for thousands of years. The Babylonians liked the number because it divides cleanly by lots of smaller numbers — 1, 2, 3, 4, 5, 6, 8, 9, 10, 12 — which makes mental arithmetic with angles easy.

So:

  • A full turn (all the way around) is 360°.
  • A half turn (a straight line) is 180°.
  • A quarter turn (a square corner) is 90°.
  • An eighth turn is 45°.

Where to spot angles

  • The corner of a book or a brick — a right angle (90°).
  • The hands of a clock at 3 o'clock — also 90°.
  • A slice of pizza cut into eighths — each slice is 45°.
  • A door open slightly — usually about 30°.

What you'll learn

Try it out