# How do you determine if a line is perpendicular, parallel, or neither?

$\text{Line 1} : y = {m}_{1} x + {c}_{1}$
$\text{Line 2} : y = {m}_{2} x + {c}_{2}$
If ${m}_{1} = {m}_{2}$ then the lines are parallel, since they both have the same rate of change and therefore will never cross.
If ${m}_{1} {m}_{2} = - 1$ then they are perpendicular.