Horizontal line test

The Horizontal line test is a simple way to see if a function is one-to-one (that is, it has exactly y-value corresponding with every x-value, or dependent value for every independent value). If a function passes the Horizontal line test, it has an inverse function. This is because the inverse of a function swaps the domain and range of the original function (the domain of the original function becomes the range of the inverse, and vice versa).