A line parallel to the has no defined slope, and a line parallel to the
has a slope of zero.

A line parallel to the "vertical line" has no defined slope, and a line parallel to the "horizontal line" has a slope of zero.

Vertical lines have undefined slope, as any two points on the line have the same x-coordinate. The equation of vertical lines are often represented by, x = a, where a is the x-intercept in (a, 0). This means that the x-coordinate will always be the same regardless of the y-coordinate paired with it. If one were to solve for the slope of vertical lines using the formula, m = (y2 - y1)/(x2 - x1), it will not produce a finite number because division by zero is an undefined operation.

Horizontal lines are often represented by, y = b, where b is the y-intercept in (0, b).
A horizontal line have a constant y-coordinate, regardless of its paired corresponding x-coordinates.