This book is a modern exploration of the feature [tense] introduced in distinctive feature theory by Roman Jakobson and colleagues (Jakobson, Fant, and Halle 1952). By proposing the feature again after years of skepticism toward its phonetic motivation, Jessen aims to provide a contribution to the integration of phonetics and phonology within linguistic theory. This thorough study of the tense/lax opposition in German obstruents is an expanded version of Jessen's 1996 Cornell University Ph.D. dissertation and is of particular significance because it reveals details and clarification of Jakobsonian concepts through personal communication with Linda Waugh, a former student of and later close collaborator with Jakobson himself. As such, the book will be of interest to phonologists, linguistic phoneticians, and those interested in the very concept of distinctive features and their role in describing the sound structure of human languages. Some background in acoustic phonetics and familiarity with experimental phonetic research is desirable to fully appreciate the power of the arguments presented in the study.