This study proposes a new view of glossing as a universal phenomenon. Starting from the Psalter, a centrepiece of devotion and education in early medieval Europe, it combines historical sociolinguistics, comparative philology, manuscript studies and cultural history in order to assess and compare the interface of Latin with Old Irish, Old English, Old Frisian, Old Saxon and Old High German within the context of its multilingual and textual culture. The close study of thirteen glossed manuscripts, such as the Anglo-Saxon Vespasian Psalter and the Old Irish Milan Glosses, reveals when and why scribes switched from Latin into the vernacular, how the vernacular was used in studying Latin, how glosses interact with construe marks and punctuation, and how such manuscripts were intended to be read in a period covering the seventh to the twelfth centuries and in an area stretching from Ireland to Central Europe. The book is an essential textbook for specialists in the growing field of glossing, and also reaches out to scholars of early medieval liturgy, education, palaeography and Christian literature.