The domain is the set of all "x" values and the range is set of all "y" values in a set of ordered pairs. Remember that ordered pairs are written as (x, y). When looking at a set of ordered pairs, find the domain by listing all the x values from the relation. Find the range by listing all the y values from the ordered pairs. Repeated values within the domain or range don't have to be listed more than once. In order for a relation to be a function, each x must correspond with only one y value. If an x value has more than one y-value associate with it -- for example, in the relation {(4, 1), (4,2)}, the x-value of 4 has a y-value of 1 and 2, so this set of ordered pairs is not a function. If each x-value corresponds with only one y-value, then the relation is a function.