The construction of the first lexicographic works in Spanish and other languages says a lot about how later dictionaries were built from a methodological perspective, especially those published by the Real Academia. After a revision of the origins of the first largest dictionary of Spanish, known as Diccionario de Autoridades (1726-39), we aim at providing examples about the criteria followed by its collaborators in the task of adding words to it, which it was justified by the language use of well known writers and works (auctores). We focus on the study of one of the auctoritates, Fuero Juzgo, a medieval codex of Spanish laws (13th century). The methodological treatment of this dictionary comes from the Italian and French lexicographical traditions (Vocabolario della Crusca and Dictionnaire de la Académie Française), although there were differences in the way of selecting the words. The concern for gathering as many words as possible leaded to the inclusion of terms of different traditions and knowledge. However, the use of contemporary editions to justify the entered words caused inconsistencies in the addition of the formal variations of selected terms, as in the case of Fuero Juzgo, and might reinforce the idea of the later orthographic reforms, already expressed in the prologue of the Diccionario.