Parallel and Perpendicular Lines
Parallel lines are coplanar lines that never intersect.
In two dimensions, parallel lines have the same slope .
The two lines
are parallel, since they both have slope .
Perpendicular lines are lines that intersect at right angles. If you multiply the slopes of two perpendicular lines, you get .
are perpendicular, since
Exception : A horizontal line and a vertical line are perpendicular, even though the product of their slopes is undefined.