Final version submitted in June 1999, published in Language 76:297-323 (2000).
In most standard dialects of English, there is a gap in the paradigm of the verb BE where we should expect to find *amn't. But how do we know that this gap exists, since learners have no positive evidence that *amn't is ungrammatical? It is even more puzzling since there is no gap when the subject is inverted (Aren't I...?). Familiar explanation for this gap fail; in particular, it cannot be the result of conservative acquisition strategies. The explanation offered here is based crucially on a combination of multiple default inheritance and function-based morphology, as embodied in word grammar. The gap is due to a `Nixon diamond' conflict between two competing values for the same morphological function required by the categories `negative' and `first-person'. The inverted form is supplied by stipulation because of the functional pressure to supply a `casual' form. The paper also considers various dialect alternatives to the Standard English pattern. The success of this explanation that language must use default inheritance, multiple non-orthogonal inheritance, and morphological functions.