The Epistle of Jerome to Pope Damasus I (Latin Epistula Hieronymi ad Damasum papam), written in 376 or 377 AD, is a response of Jerome to a epistle from Damasus, who urged him to make a new translational work of the Holy Scripture. The letter was written before Jerome started his translational work (382-405).
Jerome agreed that Old-Latin translation should be revised and corrected. There are also numerous differences between every Latin manuscript and every one looks like another version. They should be corrected on the basis of the Greek manuscripts. Jerome explained why the Old-Latin order of the Gospels (Matthew, John, Luke, Mark) should be changed into the order Matthew, Mark, Luke, John, because it is relevant for the Greek manuscripts. Jerome explained importance of the Eusebian Canons and how to use them.
The copy of letter occurs in some Latin manuscripts of the New Testament (even in Old Latin Codex Sangallensis 48). Usually it is placed at the beginning of the manuscript (e.g. Codex Sangallensis 48).