# angle

Types of angle.

Protractor.

"My geometry teacher was sometimes acute, and sometimes obtuse, but he was always right." — anonymous

In plane geometry, an angle
is the opening between two lines or two planes that meet; the word comes
from the Latin *angulus* for a sharp bend. Plane angles are measured
in degrees or radians.

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.

In spherical geometry, a **spherical angle** is that formed by intersecting
arcs of two great circles; its magnitude
is equal to that of the angle between the two planes of the great circles.

See also solid angle.

## Protractor

A protractor is an instrument for measuring angles. Usually semicircular, it is marked off in degrees along the semicircular edge.