This significant contribution to the generative theory of syntax confronts a fundamental question inherent in the study of universals of grammar: How can claims of universality be empirically verified? This book puts the question of the similarity of categories across the grammars of particular languages on a substantive empirical basis by providing a principled method of cross-linguistic comparison; a method based on equivalence classes of language-particular properties relative to a universally applicable definition. While the critical issue is general to a consideration of any syntactic category, the framework proposed is applied specifically to a category called AUX. A considerable body of evidence is introduced both for the framework and for the category through the detailed examination of a number of languages—Lummi, Egyptian Arabic, Japanese, Luiseno, German, and English. The book presents a unified exposition and statement rather than an anthology of the authors' various views on the topic of the category AUX. The major portion of the book was written by Dr. Steele.