Types of angle.
"My geometry teacher was sometimes acute, and sometimes obtuse, but he was always right." — anonymous
A right angle has exactly 90° (or π/2 radians), an acute angle less than 90°, an obtuse angle has between 90° and 180°, and a straight angle has exactly 180°. Any angle that is at most 180° is said to be convex. An angle between 180° and 360° is called a reflex angle. An angle of exactly 360° is said to be a full angle.
Complementary angles add to give 90°, supplementary angles make a total of 180°. Adjacent angles are next to, and thus share, one side.
A reentrant angle is an inward-pointing angle of a concave polygon.
See also solid angle.
A protractor is an instrument for measuring angles. Usually semicircular, it is marked off in degrees along the semicircular edge.