What is an angle?
An angle is the amount of turn between two lines that meet at a point. We see angles everywhere, such as the corner of a book or the hands of a clock. We measure angles in degrees (°) using a protractor. A quarter turn is 90°, a half turn is 180°, and a full turn all the way around is 360°.
Key idea
Acute angle: less than 90°. Right angle: exactly 90°. Obtuse angle: between 90° and 180°. Reflex angle: more than 180°.
Angles on a line and at a point
Angles on a straight line add up to 180°. Angles all around a point add up to 360°.
Worked example
Two angles sit on a straight line. One is 120°. What is the other? Angles on a line make 180°, so 180 − 120 = 60°.
Remember
- Right angle = 90°, straight angle = 180°, full turn = 360°.
- Angles on a line add to 180°.
- Angles at a point add to 360°.
- When measuring with a protractor, line up 0° along one arm and read where the other arm points.