If K is a signature, then a structure of signature K, say A, consists of the following items:
A set called the domain of A and written dom(A); it is usually assumed to be nonempty;
for each individual constant c in K, an element cA of dom(A);
for each predicate symbol P of arity n, an n-ary relation PA on dom(A);
for each function symbol F of arity n, an n-ary function FA from dom(A) to dom(A).