When you measure lengths you use different units, including millimetres, metres and kilometres. But length is not the only type of measurement you need to make when you are looking at flat shapes. Sometimes, you need to change direction, for example, if you turn a corner.
A turn between one direction and another is called an angle. You measure angles in degrees. One whole turn is 360 degrees, written as ${360^ \circ }$.
Humans have needed to measure angles for a long time. When early astronomers looked at stars in the sky, they wanted to describe their positions relative to one another. The natural way to do this was to use angles. We know that the ancient Babylonians and Egyptians divided a whole turn into 360 parts, as long ago as 1500 BC.
Why are there 360 degrees in a whole turn?
A clay tablet excavated in Shush, in what is now modern-day Iran, shows that Babylonians divided a whole turn into 360 units. One reason could be that many simple fractions of a whole turn of ${360^ \circ }$, including $\frac{1}{2}$ , $\frac{1}{3}$ , $\frac{1}{4}$ , $\frac{1}{5}$ and $\frac{1}{6}$ can be written as a whole number of degrees. It may also be because there are approximately 360 days in a year.
The angles of a triangle
The sum or total of the angles of a triangle is always ${180^ \circ }$.

In this unit you will learn about other angle facts and use them to solve problems.

This diagram shows parts of lines between two points, $AB, AC$ and AD.
You can label a line segment by writing down the letters of the points at each end of the line.
You can write the points in any order: AB and BA are two different ways to label the same line segment.
If you look again at the diagram, you can see several angles.

The angle between AB and AC is called angle BAC or angle CAB. The letter of the point of the angle is always in the middle.
Here is part of the diagram again.
There are two angles at A between AB and AC.

One is an acute angle. The other angle is more than two right angles.
An angle that is more than two right angles is called a reflex angle.
Normally, if you refer to 'angle BAC, you would mean the smaller of the two angles. If you want to refer to the other one, you must call it 'reflex angle BAC'.