# Parallel and perpendicular lines

**Parallel lines**

The general rule is that if two lines have with the same gradient they are parallel.

This can be calculated using the *y* = *mx* + *c* equation as discussed in the previous section.

**Perpendicular lines**

Perpendicular lines are so called as they have a product of -1. Therefore if the gradient of a line is *m*, the the gradient of the line perpendicular to it is .