Judah ben David Hayyuj (Arabic: أبو زكريا يحيى بن داؤد حيوج Abu Zakariyya Yahya ibn Dawūd) was a Spanish-Jewish grammarian; born in Fez, Morocco, about 945. At an early age he went to Cordoba, where he seems to have remained till his death, which occurred about 1000 CE.

Career
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.
As a supplement to this treatise he wrote a second, which he called the Kitab al-Af'al Dhawat al-Mathalain (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, together with their various forms occurring in the Bible. Besides the two treatises on verbs Hayyuj wrote Kitab al-Tanqit (The Book of Punctuation). This work, probably written before his two chief treatises, is an attempt to set forth the features underlying the Masoretic use of the vowels and of the word-tone. In this work he deals chiefly with nouns, and its purpose is more of a practical than of a theoretical character.
A fourth work, the Kitab al-Natf (The Book of Extracts), is known to have been written by Hayyuj, but only a fragment, unpublished as of the beginning of the 20th century, and a few quotations by later authors have survived. This was a supplement to his two grammatical works on the verb, and in it he noted the verbs omitted by him in the former treatises. In doing this he anticipated in a measure ibn Janah's Mustalhaq 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 Bible.