This is accomplished largely through rule change of a very restricted sort: optional rules become obligatory and rules generalize. Thus part of the answer to the question of why language changes is that since optional rules are less highly valued than obligatory ones, the language learner will - whenever possible - reduce the level of optionality, either by changing the status of the rule to obligatory or through elimination of the rule from the grammar of the language.

In their descriptive and historical work, the authors depend heavily on recent advances in phonological theory. Crucial to the study are certain insights of metrical theory and of autosegmental phonology.

Archangeli's theory of underspecification and the Principle of Strict Cyclicity as developed over the years by a number of linguists (including Kean, Kiparsky, Mascaro, Halle, and Mohanan) also figure prominently in this study. In addition, the authors make two basic contributions to phonological theory: the Structural Hierarchy according to which syllables and sequences of syllables are universally ranked and a convention of rule formalization for collapsing into a single rule optional and obligatory rules that overlap significantly in their context of application. Both of these phonological theoretic innovations play a significant role in the questions of language change that are at the center of this study.

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