Parallel lines are coplanar lines that never intersect.
In two dimensions, parallel lines have the same slope.
The two lines
y = 3x + 8
y = 3x – 4
are parallel, since they both have slope 3.
Perpendicular lines are lines that intersect at right angles. If you multiply the slopes of two perpendicular lines, you get –1.
are perpendicular, since
Exception: A horizontal line and a vertical line are perpendicular, even though the product of their slopes is undefined.