Naming and classifying angles
Angles are named or classified according to their size.
Acute angles are smaller than 90°.
Obtuse angles are bigger than 90°, but smaller than 180°.
Reflex angles are bigger than 180°.
A right angle is exactly 90°. If two lines are at right angles, they are perpendicular.
A straight line is 180°, and a full turn is 360°.
Angles are usually measured in degrees (at least for GCSE); remember that this is different to °C or °F, which are used to measure temperature.