The employment of diverse texts as sources of lexicographic material to exemplify and attest the use of the lemmata is one of the most recognizable traits of the first dictionary published by the Spanish Royal Academy. This Master’s Thesis aims at two main objectives. Firstly, the text aspires to contribute to the general study on the specialized lexicon lemmatized in the first academic dictionary, thus concentrating on the legal vocabulary. Secondly, it examines the sources of the lexicographic material while focusing on the employment of the medieval legal code Fuero Juzgo as one of the sources. By examining both the implementation of this code as a source as well as the treatment of the legal vocabulary it exemplifies, this work provides an insight into both semantic and formal genesis of the lexical units tied to the domain of law through time but also depicts different types of aspects (historical, linguistic, lexicographical, etc.) which, in one way or another, have influenced this process.