 Although Class Comprehension and Limitation of Size appear to tell us exactly what classes there are and what sets there are, more axioms are required to make our universe large enough. These can be taken to be the axioms of Z (other than extensionality and choice, which are not needed): the sethood of pairs of sets, unions of sets, power sets of sets, and the existence of an infinite set are enough to give us the world of ZFC. Foundation is usually added. The resulting theory is a conservative extension of ZFC: it proves all the theorems of ZFC about sets, and it does not prove any theorem about sets which is not provable in ZFC. For those with qualms about choice (or about global choice), Limitation of Size can be restricted to merely assert that the image of a set under a class function is a set.