Notice now two cases in which the straight lines of the same origin are contained in the same line. In such cases, angles are also formed.

## Null angle and one turn angle

- The half straight coincide. We have there
**null angle**and the angle of**a turn.**

## Shallow or half turn angle

- The half straight do not match. We have two
**shallow angles**or of**U turn.**

We can then establish that:

**Angle**is the region of the plane bounded by two semi-straights that have the same origin. Next: Measuring an Angle