The variable lexical phonology proposed in Guy (1991) predicts an exponential relationship among rates of retention in word classes of different derivational histories. A class of words that satisfies the structural description of a variable rule at an early lexical level of derivation will undergo multiple operations of the rule and, therefore, exponentially reduced rates of retention, compared to a class of forms that only satisfies the structural description of the rule at the end of the derivation and thus is subject to its operation only once. For the case of English -t,d deletion, it is postulated that monomorphemic words (e.g., mist) are exposed to the deletion rule three times in a derivation, whereas semiweak past tense forms (e.g., left) are exposed twice, and regular past tense forms (e.g., missed) undergo the rule but once.
The present article explores the consequences of this model for other variable constraints on a rule, such as the preceding and following segment constraints on -t,d deletion. Word-internal constraints, because they are present throughout the derivation, are shown to have quantitatively different patterns than external constraints, as the latter affect the rule only in its final, post-lexical operation. Four specific quantitative predictions are derived from the model to elucidate this distinction between internal and external constraints, and empirical data are presented to confirm the predictions.