Horizontal Line Test Problem: The vertical line test of Problem 39 in Section $1-2$ helps you see graphically that a relation is a function if no vertical line crosses the graph more than once. The horizontal line test allows you to tell whether or not a function is invertible. Sketch two graphs, one for an invertible function and one for a noninvertible function, that illustrate the following test. The Horizontal Line Test If a horizontal line cuts the graph of a function in more than one place, then the function is not invertible because it is not one-to-one.