parallel

In a two-dimensional plane, two lines are parallel if they do not intersect. (If the lines begin or end in a specific vertex, they have to be extended to check that they do not intersect in any place after being extended.)

The first picture shows a red line and a blue line. The red line is parallel to the blue line.

We include arrows pointing in the same direction on lines that are parallel. The second picture shows this.

Angles also use parallel lines:

• alternate angles

• corresponding angles

• interior angles.