Over the vocabulary of CNF instances, an instance has width at most three when no clause has four distinct literal occurrences. It is a yes-instance of 3-DNF-TAUT when it has width at most three and, read as a formula in disjunctive normal form, is a tautology; it is a yes-instance of 3-UNSAT when it has width at most three and, read as a formula in conjunctive normal form, is unsatisfiable. Each problem is the decision problem of the structures isomorphic to such an instance. The width bound is part of both problems, so 3-UNSAT is not the complement of 3SAT, which wide instances also satisfy.