# Angle

An **angle** is the space between two lines that meet each other. When two lines meet at a point, an angle is formed. The two lines are called the *arms* of the angle.

To measure the size of an angle, we use degree. Degree is a standard unit. Degree is denoted by the symbol '°'. A degree can be divided into 60 minutes (1° = 60'), and a minute can be also be divided into 60 seconds (1' = 60"). In mathematics, angles are usually measured in radians.

## Types of angles

An acute angle is an angle less than 90°. A right angle is an angle equal to 90°. An obtuse angle is an angle greater than 90° but less than 180°. A straight angle (or straight line) is an angle equal to 180°. A reflex angle is an angle greater than 180° but less than 360°.

Supplementary angles are two angles with the sum equal to 180°.

Two angles that sum to one right angle (90°) are called complementary angles.

Two angles that sum to one full circle (360°) are called explementary angles or conjugate angles.

People usually use a protractor to measure and draw angles. Sometimes, people use an angle ruler or a 360° protractor to measure angles.