Rule

Perpendicular Lines

Lines that intersect at right angles are called perpendicular lines. All vertical lines are perpendicular to all horizontal lines. Lines are perpendicular if and only if the product of their slopes, $m_{1}$ and $m_{2},$ is $- 1 .$

$m_{1} \cdot m_{2} = - 1$

This graph shows one pair of perpendicular lines.

For the product of two slopes to equal $- 1,$ the slopes must be opposite reciprocals. For example, $m_{1} = - \frac{2}{3}$ and $m_{2} = \frac{3}{2}$ are opposite reciprocal slopes.