Hayyuj announced that all Hebrew stems consist of three letters, and maintained that when one of those letters was a "vowel letter," such a letter could be regarded as "concealed" in diverse ways in the various verbal forms. To substantiate his theory he wrote the treatise upon which his reputation chiefly rests, the "Kitab al-Af'al Dhawat Huruf al-Lin" (The Book of Verbs Containing Weak Letters). The treatise is in three parts: the first is devoted to verbs whose first radical is a weak letter; the second to verbs whose second radical is weak; and the third to verbs whose third radical is weak. Within each division he furnishes what he considers a complete list of the verbs belonging to the class in question, enumerates various forms of the verb, and, when necessary, adds brief comments and explanations. Preceding each division the principles underlying the formation of the stems belonging to the division are systematically set forth in a series of introductory chapters.