The Antiphonary of Leon, "jewel of Latin antiphonaries" (dom Louis Brou), is the only complete one of the ancient Hispanic liturgy. Preserved in the Cathedral of Leon and dated to the tenth century, it is introduced by twenty-seven folios that most certainly belong to the original manuscript. Still largely unknown despite their great richness, these folios contain literary and liturgical texts, a calendar, miniatures, tables and computus wheels, as well as a computus treatise. The edition of these pieces - with the exception of the treatise - and their multidisciplinary analysis (codicological, paleographic, iconographic, literary, musicological, liturgical and computistic) allow for an in-depth study of the religious life and culture of this period, the importance of the Visigothic heritage, the influences from the ultra-Pyrenean world, as well as the origins and reworkings of this codex. These introductory folios constitute both the gateway and the key to reading a manuscript whose history lies at the heart of the major issues of its time: produced for the monastery of San Cipriano del Condado, it was quickly transferred to the monastery of Santiago de Leon and then to the chapter of the cathedral. Although close to the centres of power it contains the signatures of King Ferdinand I and his sons it sank into oblivion when his son Alfonso VI agreed in 1080 to renounce the Hispanic liturgy and to adopt, at the request of Pope Gregory VII, the Roman rite. This study contributes to its rediscovery.