Definition: An angle whose measure is exactly 180° - a straight line

Try this
Adjust the angle below by dragging an orange dot and see how the angle ∠ABC behaves.
Note that it is a Straight Angle only when it is exactly 180°

Straight (or Flat) angles are formed when the legs are pointing in exactly opposite directions. The two legs then form a single straight line through the vertex of the angle. The measure of a flat angle is thus always 180°

Acute angle Less than 90° |
Right angle Exactly 90° |
Obtuse angle Between 90° and 180° |

Straight angle Exactly 180° |
Reflex angle Between 180° and 360° |
Full angle Exactly 360° |

- Vertical angles
- Complementary angles
- Supplementary angles
- Linear pair
- Adjacent angles
- Corresponding angles
- Alternate interior angles
- Alternate exterior angles
- Interior angles of a transversal
- Exterior angles of a transversal

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