Parallel and Perpendicular Lines
Parallel lines are coplanar lines that never intersect.
In two dimensions, parallel lines have the same slope.
Example:
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.
Example:
The lines
are perpendicular, since
Exception: A horizontal line and a vertical line are perpendicular, even though the product of their slopes 
is undefined.
