# Exterior angles formed by Parallel lines and their Transversal

The external angles, formed by the intersection of two parallel lines and their transversal are called the exterior angles of parallel lines and their transversal.

## Exterior angles

Four exterior angles are formed geometrically when two parallel lines are intersected by their transversal.

$1. \,\,\,\,\,\,$ $\angle BPX$ is first exterior angle

$2. \,\,\,\,\,\,$ $\angle XPA$ is second exterior angle

$3. \,\,\,\,\,\,$ $\angle CQY$ is third exterior angle

$4. \,\,\,\,\,\,$ $\angle YQD$ is fourth exterior angle

The four angles are formed externally when two parallel lines are cut by a straight line. So, they are called as exterior angles.

### Property

The exterior angles which appear in opposite side alternatively are equal geometrically when a pair of parallel lines cut by a transversal.

$1. \,\,\,\,\,\,$ $\angle BPX = \angle CQY$

$2. \,\,\,\,\,\,$ $\angle XPA = \angle YQD$

The property of exterior angles is often useful to us while studying the geometrical mathematics and its related subjects.