Geometry is a branch of math that, at its basic level, is concerned with describing shapes and space. So it helps to be familiar with basic definitions and vocabulary relating to angles, lines, and shapes. As you study this section, add new definitions to your notes.
Angles and Lines
Angles measure the amount of turning from one position to another. Imagine looking straight ahead and then turning around until you return to your starting position. The angle you have turned through is a full turn. If you turn so that you are facing in the opposite direction, you will have made half of a full turn. If you turn from looking straight ahead to facing either directly to your right or directly to your left, you will have made a quarter turn.
Smaller turns can be described by splitting the full turn, that is a circle, into 360 equal intervals known as degrees. So one full turn is the same as 360 degrees, which is written as 360º. This means that half a turn is 180º and a quarter turn is 90º as shown below. [ Why 360º? We inherited this from the Babylonians! Their counting system was based on 60 and they first used degrees in astronomy. ]
An angle of 90º is known as a right angle.
Lines which are at right angles to each other are said to be perpendicular. On a diagram, right angles are denoted by a small square drawn at the angle. Angles can be measured using an instrument called a protractor.