What is an angle?
An angle is the amount of turn between two lines that meet at a point. We measure the size of an angle in degrees (°).
Key idea
Acute angle: less than 90°. Right angle: exactly 90°. Obtuse angle: more than 90° but less than 180°. Straight angle: 180°.
Turns and right angles
A quarter turn is one right angle (90°). A half turn is two right angles (180°). The corner of a book or a door is a right angle.
Worked example
A clock hand turns from one right angle and then 25° more. What is the whole angle?
90° + 25° = 115°. Because 115° is more than 90° but less than 180°, it is an obtuse angle.
Remember
- Smaller than 90° is acute; exactly 90° is a right angle.
- Between 90° and 180° is obtuse.
- We write angle sizes with the degree sign, like 90°.