 All of the formal languages sketched above have only one sort of variable. These are sometimes called first-order variables. Each interpretation of the language has a domain, which is the range of these first-order variables. It is what the language is about, according to the given interpretation. Second-order variables range over properties, sets, classes, relations, or functions of the items in that domain. Third-order variables range over properties, classes, relations of whatever is in the range of the second-order variables. And it goes on from there.