What's the set of all the things in my universe that are not, that are not in A? Now let's make things a little bit more concrete by talking about sets of numbers. Once again, our sets, we could have been talking about sets of TV personalities or sets of animals or whatever it might be, but numbers are a nice, simple thing to deal with. And let's say that our universe, our universe that we care about right over here, is the set of integers. So our universe is a set of integers. So I'll write U, capital U, is equal to the set of integers. And this is a little bit of an aside, but the notation for the set of integers tends to be a bold Z.