A half turn is 180° and is known as a straight angle (or angle on a straight line). A three-quarter turn is 270°. An acute angle is less than 90°. An obtuse angle is between 90° and 180°.

Just have a look at the edges of your desk; the hands of a clock or the keys on your computer. There are examples everywhere. Angles are measured using degrees, ranging from 0 all the way to 360 ...