Vertical line test

In mathematics, the vertical line test is a visual way to determine if a curve is a graph of a function or not. A function can only have one output, y, for each unique input, x. If a vertical line intersects a curve on an xy-plane more than once then for one value of x the curve has more than one value of y, and so, the curve does not represent a function. If all vertical lines intersect a curve at most once then the curve represents a function.^[1]

The vertical line test, shown graphically. The abscissa shows the domain of the (to be tested) function.

See also

• Horizontal line test