The "vowel shift" alternations in English are not explained by the traditional approach to vowel features, in which vowels are described by the height and retraction of the tongue. In order to understand the alternations, äs well äs the historical processes that gave rise to them (the Great Vowel Shift) and similar processes in other languages a different approach to vowel features is needed. This study discusses such an approach. Combining earlier approaches to the phonetics and phonology of vowels with the theory of feature geometry, it is proposed that there are three articula tor tiers that define vowel articulations: Labial, Coronal, and Mandibular, the latter deüning degree of aperture. In addition, tenseness is described äs a narrowing of the air passage by muscular action in the tongue. Under such a system, the alternations seen in the English vowel system are accounted for by very natural rules, instead of the conventional alpha-switching approach. The historical Great Vowel Shift can also be better explained by such an approach.It is the goal of generative grammar to provide explanations for grammatical processes. The present study examines a phonological phenomenon which we shall refer to here äs shifty vowels. Shifty vowels are vowels which undergo a rule-governed change in quality under certain conditions, most frequently triggered by tenseness. The best-known case of shifty vowels, and the one from which the term is transparently derived, is the phenomenon of Vowel Shift in Modern English, where this term can be used ambiguously to refer either to the historic Great Vowel Shift of Early Modern English or to the long-short alternations of present-day English which are the synchronic reflex of this historic process. Vowel Shift has received much attention in the literature of generative phonology, but despite many technical improvements in the analysis we are no closer to an explanation today than we were with the publication of Chomsky and Halle (1968), henceforth SPE.The explanation must lie in the nature of the phonological features which define vowels. Most contemporary studies of the phonology of vowels assume the feature system of SPE, with slight Foiia Lingüistica XXV/3-4 0165^004/91/25-483 $ 2,-