A major insight of Optimality Theory (Prince & Smolensky 1993) is that grammatical constraints are ranked and violable. These ranked constraints evaluate an infinite set of candidate output forms, of which the optimal one is the actual output. The winning candidate is a compromise between the potentially conflicting demands that grammatical constraints impose. A question that the OT literature has only rarely addressed is how ungrammaticality arises if constraints are violable and violation does not entail ungrammaticality.

In this paper, we point to some shortcomings of the only existing proposal to deal with ungrammaticality in OT, the special constraint MParse (Prince & Smolensky 1993). We propose a restructuring of the architecture of the OT constraint system that overcomes these shortcomings. We show that one of the great strengths of OT, that of separating well-formedness from the repair strategies to arrive at well-formed structures, is a weakness in dealing with absolute ungrammaticality. MParse forces us to consider what repairs might have been employed to fix up an ill-formed string. However, as we show in several cases, absolute ungrammaticality should be considered separately from the issue of possible repairs. Ungrammaticality results when the optimal form a grammar can produce still fails to satisfy a constraint governing ungrammaticality. MParse, as a component of Eval, requires us to evaluate multiple candidates, hence multiple repairs, simultaneously. We demonstrate that existence of a repair shown by particular alternations in a language (for example to avoid impermissible coda clusters) does not mean that the same repair will be available as a measure of last recourse to save an otherwise ungrammatical form (for example, to augment a subminimal form).

We propose to add a non-optimising constraint component called Control, which contains only those inviolable constraints that cause ungrammaticality rather than repair. If the winning candidate from Eval, the usual ranked and violable constraint component, satisfies all the constraints in Control, it is a grammatical output. If it violates a constraint in Control, no grammatical output is possible. This approach is empirically superior to MParse, and it also makes clearer a crucial distinction between two kinds of inviolable constraints that has not enjoyed much explicit attention in the literature. Inviolable constraints in Eval outrank all potentially conflicting constraints and cause repairs or block otherwise general alternations. Inviolable constraints in Control cause ungrammaticality, never repair.Two new developments in OT might possibly have a bearing on the success or failure of MParse. The first of these is McCarthy's (1998) Sympathy Theory. The second is Sprouse's (1997) Enriched Input Theory. Both of these models are in the early stages of development. There are no published references as yet for either. Furthermore, McCarthy (1999) is a revision of Sympathy Theory designed to reduce its currently excessive formal power. Since the proper form of these theories is as yet unclear, we refrain from discussing them here. To the best of our knowledge, however, our Control proposal is fully compatible with both.