Two lines in a coordinate plane are perpendicular if they intersect at a right angle. All vertical lines are perpendicular to all horizontal lines. Two lines written in slope-intercept form, $y=mx+b,$ are perpendicular if and only if the product of their slopes equal $\text{-}1.$

When the product of two slopes equals $\text{-}1,$ the slopes are said to be opposite reciprocals. For example, $m_1=\text{-}\frac{2}{3}$ and $m_2=\frac{3}{2}$ are opposite reciprocal slopes.