A function is defined only if for each value of x, there is only one equivalent value of y. A function is called one-to-one if each value of y is related to only one value of x. Graph 1 is a function, but not a one-to-one function, as for example, y = 3 for x = -3.5 and x = 5, that is, a value of y is related to more than one value of x. Graph 2 is not a function, as a single value of x is related to more than one value of y. Graph 3 is a function, but not one-to-one, for the same reason as graph 1, as for example, y = 1 for x = -1 and x = 1. Graph 4 is a one-to-one function, as each value of y is related to only one value of x.