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
Kitāb al-Af‘āl Dhawāt Ḥurūf al-Līn "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 class' verbs, enumerates various verb forms, 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 outlined in a series of introductory chapters. As a supplement to this treatise he wrote a second, which he called the
Kitāb al-Af‘āl Dhawāt al-Mithlayn "The Book of Verbs Containing Double Letters", and in which he points out the principles governing the verbs whose second and third radicals are alike. He furnishes a list of these verbs and their various forms occurring in the
Bible. Besides the two treatises on verbs, Hayyuj wrote
Kitāb al-Tanqīṭ "The Book of
Punctuation". This work, probably written before his two chief treatises, attempts to set forth the features underlying the Masoretic use of the vowels and tone. In this work, he deals chiefly with nouns, and their purpose is more of a practical than of a theoretical character. A fourth work, the
Kitāb al-Nutaf "The Book of Extracts," is known to have been written by Hayyuj, but only a few fragments and quotations by later authors have survived. This was a supplement to his two grammatical works on the verb, and he noted the verbs he omitted in the former treatises. In doing this he anticipated in a measure
Jonah ibn Janāḥ's
Mustalḥaq, which was devoted to this very purpose. He arranged and discussed the verbal stems in question, not alphabetically, but in the order in which they occur in the
Hebrew Bible. An edition consisting of all discovered fragments was published by Nasir Basal in 2001. ==Influence==