Angles {acute angle}| can be less than 90 degrees.
Two angles {complementary angle}| can add to 90 degrees.
Two angles {corresponding angle, plane} can have parallel corresponding sides.
One ray {initial side} can be stationary and one ray {terminal side} can rotate, so angle {directed angle} changes.
Two angles {equal angle} with parallel sides are equal.
Polygon sides make angles {exterior angle} outside polygon vertices. External angle is 360 degrees minus internal angle.
Two polygon sides make an angle {interior angle} {internal angle} inside a polygon vertex.
Angles {obtuse angle}| can be between 90 and 180 degrees.
Angles {quadrantal angle} can be (n * pi) / 2 radians, where n = 0, 1, 2, ..., for 0, 90, 180, ... degrees.
Angles {reflex angle, size} can be greater than pi radians and less than two times pi radians.
Angles {straight angle} can be pi radians.
Two angles {supplementary angle}| can add to 180 degrees.
Two lines intersect to form opposite angles {vertical angle}.
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Date Modified: 2022.0225