Slope-Intercept Form
Graphing Vertical Lines
If horizontal lines occur when there is no rise, when do vertical lines occur? Vertical lines rise, but do not run. The slope formula for a vertical line is 
, which is undefined. So the equation of a vertical line does not fit within the parameters of the slope-intercept equation, because the slope is undefined. Consider the vertical line below:
This line contains the points (4, -1), (4, 0), (4, 1), (4, 2), etc. Notice that the x-value is always 4. The equation of this line is x = 4. The equation of a vertical line will always have the form x = "a constant". For example x = -2, will have a graph of a vertical line crossing the x-axis at -2.