Classifying angles by size
Every angle belongs to one of five classes by size. Naming them is the foundation of all later geometry.
Five types
| Type | Size | Example |
| Zero | 0° | Both arms coincide |
| Acute | between 0° and 90° | 45°, 60°, 85° |
| Right | exactly 90° | Corner of a paper, square |
| Obtuse | between 90° and 180° | 100°, 135°, 170° |
| Straight | exactly 180° | A half-line |
| Full | 360° | A complete circle |
The boundary values (0°, 90°, 180°, 360°) have special names. Everything between them is named after the nearest boundary.
Spotting tips
- Right angle = corner of a paper, book, tile. Use a corner to compare.
- Acute looks like a "point" — the arms are close.
- Obtuse looks "spread out" — the arms are far apart but not in a line.
- Straight = the angle along a line.
Example
An angle measures 127°. What type is it?
- 127° is more than 90° and less than 180° → obtuse.
Common traps
- A right angle is exactly 90°, not "almost 90°".
- Don't mix up obtuse and straight. 179° is still obtuse.
- Above 180° (up to 360°) the angle is called reflex.