Among the precious artifacts included in the Jewish Museum's exhibition on Italian-Jewish life is a work of singular importance. It is a lexicon of rabbinic literature called the Arukh, written by Nathan ben Jehiel of Rome (1035-c.1110) at the beginning of the twelfth century (cat. no. 61). Why so modest a work, a mere dictionary of words and phrases, should elicit special attention requires some elucidation.