Sībawaihi has occasionally been criticized for his obscurity and inconsistency. The former may be conceded, since it pertains only to his style and presentation, which undoubtedly make heavy demands on the reader, but the second criticism is an attack on Sībawaihi's linguistic reasoning and needs to be refuted. In the following pages we shall examine the background and use of the phrase ‘ishrūna dirhaman ‘20 dirhams’ as a locus probans in more than a score of grammatical arguments in the Kitᾱb, to which must be added a dozen or so other cases involving the use of similar expressions and grammatical principles. Two results should emerge: first we should gain the impression that the Kitᾱb is a work of the utmost coherence and consistency (with due allowance for the date of its composition), and second we shall uncover certain features of Slbawaihi's grammatical theory which appear to have been ignored by later grammarians.