An angle is how far something turns. Turn the arm, name the angle, measure it with a protractor and find the missing angle.
Smaller than a right angle: less than 90°.
Acute
Smaller than a right angle: less than 90°.
Right angle
A square corner, a quarter turn: exactly 90°.
Obtuse
Bigger than a right angle but less than a straight line: between 90° and 180°.
Straight
A straight line, a half turn: exactly 180°.
Reflex
Bigger than a straight line: between 180° and 360°.
Full turn
All the way round: 360°.
Angles on a straight line add up to 180°.
Angles round a point add up to 360°.
The angles in a triangle add up to 180°.
An angle measures a turn: how far one arm has turned away from the other. We measure angles in degrees (°). A full turn is 360°, a half turn 180° and a quarter turn, a right angle, 90°.
Put the centre of the protractor on the corner of the angle and line up the 0 line with one arm. Then count up from that 0 to the other arm. Protractors have two scales, so always use the one that starts at 0 on your arm.
An acute angle is less than 90°, a right angle is exactly 90°, an obtuse angle is between 90° and 180°, a straight angle is 180° and a reflex angle is more than 180° but less than 360°.
Children meet right angles and turns at about 7 or 8, name acute and obtuse angles at about 8 or 9, and measure with a protractor and find missing angles at about 9 to 11.