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°.

Geometry! HA! Who needs it ... But a rhombus has matching pairs of opposite angles. Two are acute angles, smaller than 90 degrees. And two are obtuse angles, bigger than 90 degrees.