Names of Angles
List and definition of individual names for the different kinds of angles. If for one angle a span between two values is given, both limiting values don't belong to that kind of angle.
- 0° zero angle
- 0° - 90° acute angle
- 90° right angle
- 90° - 180° obtuse angle
- 180° straight angle
- 180° - 360° reflex angle
- 360° full angle
Oblique are angles which are not integer multiples of 90°. Zero and perigon angle point to the same direction. The straight angle points to the opposite direction. The right angle is perpendicular to the original direction.
The names of angle types are primarily derived from descriptive geometric or linguistic descriptions of their shape and effect. The word angle itself derives from the Latin angulus and originally referred to a bend or a corner.
The zero angle describes the case where there is no opening between the two sides. Both sides coincide. Similarly, a full angle or perigon angle is a complete rotation around a point, returning to the starting direction. Both look the same but do not necessarily mean the same thing.
The acute angle owes its name to its narrow, pointed opening. It is smaller than a right angle. In contrast, the obtuse angle is more open and appears blunt because its apex is wider than that of a right angle.
The right angle gets its name from everyday language: right originally meant straight, upright, or proper. A right angle is perpendicular and forms the basis for many constructions in geometry, engineering, and architecture.
The straight angle corresponds to a fully straight line. The two sides lie on a straight line but point in opposite directions. It thus marks a half turn.
A reflex angle is larger than an obtuse angle and exceeds 180 degrees. It is formed when the angle is rotated beyond the straight angle until it is almost a full 90-degree angle.
Angles can also be negative if the direction of rotation is relevant. However, the terms used here refer to the absolute value of the angle.