 A Henkin model lets a second-order variable \(X^n\) range over some subset of the set of \(n\)-tuples of members of the domain \(D\). Thus a Henkin model consists of a domain \(D\) of individuals, a domain \(D^n_2\) of sets of \(n\)-tuples of objects in \(D\) for each \(n \leq 1\), and an interpretation function \(I\). To make sure that every instance of second-order comprehension is validated in the model, we require each \(D^n_2\) to contain every set of \(n\)-tuples of members of \(D\) that is definable by a formula \(A\) of the language.