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
Word
What it means
vertex
the 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°.
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°.
