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°.

An acute angle is very easy to spot ... any angle that is bigger than a right angle but less than a straight, flat line (91-179 degrees – remember them as obese or fat angles).