A vertical line is a line that runs parallel to the y-axis. A vertical line is drawn in the coordinate plane below.

Notice that the line passes through all points where the x-coordinate is 3. This means that the same x-value corresponds to infinitely many y-values. Therefore, this line does not represent a function. In fact, no vertical line is a function. Similar to horizontal lines, vertical lines take the following form.

Here, a is the x-intercept. The rule for the line above is x=3.