Geometry

Angles made by parallel lines

Parallel lines are straight lines which have the same direction and so never meet. Parallel lines are marked with an arrow as shown below.

Two parallel lines with arrows pointing left to right

In diagrams, parallel lines can form:

  • corresponding angles
  • alternate angles

Corresponding angles

These are sometimes known as 'F' angles.

The diagram below shows parallel lines being intersected by another line.

Two parallel lines with a gradient line crossing them with symbols showing equal angles

The two angles marked in this diagram are called corresponding angles and are equal to each other.

Two parallel lines with a gradient line crossing them at an angle of 100°

The two angles marked in each diagram below are called alternate angles or Z angles. They are equal.

Z shape with two parallel lines connected by a diagonal line with equal angles

Z shape with two parallel lines connected by a diagonal line with equal angles of 63°

Vertically opposite angles

Vertically opposite angles are so called because they are opposite each other at a vertex.

X shape with overlapping diagonal lines creating equal angles

X shape with overlapping diagonal lines creating equal angles of 35°

The two angles marked in each of these diagrams are called vertically opposite angles and are equal.