In mathematics, the horizontal line test is a test used to determine if a function is injective, surjective or bijective.

Suppose there is a function f : X → Y with a graph., and you have a horizontal line of X x Y : .

• If the function is injective, then it can be visualized as one whose graph is never intersected by any horizontal line more than once.
• Iff f is surjective any line will intersect the graph at least at one point
• iff f is bijective any line will intersect the graph at exactly one point.