Slope of a Line
If l and n are two different lines on the same plane, then l and n may or may not intersect. If l and n do not intersect, then l and n are said to be parallel. Parallel lines have the same steepness, as shown in the figure below.
If two lines l and n intersect and form a 90° angle, then l and n are said to be perpendicular. Refer to the figure below.
Note that on a plane, a horizontal and a vertical line are always perpendicular to each other. When none of two perpendicular lines is vertical or horizontal, such lines have slopes that are negative reciprocals of each other.
