We will mostly be concerned with mathematical functions. Suppose we have a function called f. Now, f is a rule that associates the domain, D, with the range, R. Let's let x represent one element of the domain. Thus, if D is the set of all whole numbers, x could be any whole number, say 5. Now, when we insert this x into f we get a single value, called f(x), back out. This tells us that the point (x,f(x)) is one piece of our function. We write the point in the form of an ordered pair to remind us that x is in the domain and f(x) is an element of the range. We say that f is a one variable function, because we only need to plug in one variable, or value from the domain, at a time in order to get an element of the range out of the function.