This would be the set of rational numbers. And you might say, why Q for rational? Well, there's a couple of reasons. One, the R is already taken up. And Q for quotient, a rational number can be expressed as a quotient of two integers. And we just saw you have your Z for Zoll, for Zoll or integers, the set of all integers. So our universal set, the universe that we care about right now is integers.