The “weak” triliteral stems of Standard Arabic have been studied in considerable detail by Arab as well as Western grammarians. In most cases, however, the treatment has been largely limited to listing and classifying the primary data, which satisfies observational adequacy but fails to meet the essential requirements of descriptive adequacy, simplicity and generality. In a few cases rules have been proposed, but these rules fall short of expectations: on the whole, they strike the user as complex, arbitrary and unmotivated. Drawing on the theory of Generative-Transformational Grammar, I have formulated a small set of rules which seek to satisfy all of the three essential requirements, and thus render the subject more palatable to scholars as well as learners of Standard Arabic.