 There are at least two other kinds of definition in model theory besides these two above. The third is known as interpretation (a special case of the interpretations that we began with). Here we start with a structure \(A\), and we build another structure \(B\) whose signature need not be related to that of \(A\), by defining the domain \(X\) of \(B\) and all the labelled relations and functions of \(B\) to be the relations definable in \(A\) by certain formulas with parameters. A further refinement is to find a definable equivalence relation on \(X\) and take the domain of \(B\) to be not \(X\) itself but the set of equivalence classes of this relation. The structure \(B\) built in this way is said to be interpreted in the structure \(A\).