This article examines the internal structure of triconsonantal roots in Tashlhiyt Berber. It is proposed that these roots have a binary-branching head-complement structure, built upon the sonorant and the segment immediately to its left. Evidence for this structure is provided by the imperfective formation. It is argued that only roots that display such a structure undergo gemination in the imperfective. This permits an account for a number of forms that are traditionally ascribed to lexical idiosyncrasy, including verbs that are made up entirely of obstruents and those whose only sonorant is in initial position.