In this paper,(1) I will restrict the term "linguistic theory" to systems of hypotheses concerning the general features of human language put forth in an attempt to account for a certain range of linguistic phenomena. I will not be concerned with systems of terminology or methods of investigation (analytic procedures). The central fact to which any significant linguistic theory must address itself is this: a mature speaker can produce a new sentence of his language on the appropriate occasion, and other speakers can understand it immediately, though it is equally new to them. Most of our linguistic experience, both as speakers and hearers, is with new sentences; once we have mastered a language, the class of sentences with which we can operate fluently and without difficulty or hesitation is so vast that for all practical purposes (and, obviously, for all theoretical purposes), we may regard it as infinite. (1)This work was supported in part by the U.S. Army Signal Corps, the Air Force Office of Scientific Research, and the Office of Naval Research, and in part by the National Science Foundation (Grant G-13903). The account of linguistic structure sketched below in part incorporates, and in part developed in response to many stimulating ideas of Zellig Harris and Roman Jakobson. Its present form is to a large extent a product of collaboration over many years with Morris Halle, to whom (along with Paul Postal and John Viertel) I am indebted for much helpful criticism of this paper. For references, see the bibliography at the end of the paper.