The chant that is now called "Old Roman" comes primarily from a small number of sources, including three graduals and two antiphoners from between 1071 and 1250.
In order to consolidate ecclesiastical power and strengthen their political ties to the power of the Roman church, the Franks, especially under the Carolingian rulers Pepin and Charlemagne, brought this older Roman chant north.
There it was subsequently modified, influenced by local styles and Gallican chant, and categorized into the system of eight modes.
For example, the Credo was added to the Roman rite at the behest of the German emperor Henry II in 1014.
The split between Gregorian and Old Roman appears to have taken place after 800, since the feast of All Saints, a relatively late addition to the liturgical calendar, has markedly different chants in the two traditions.
In addition to the similarities in texts noted above, corresponding Old Roman and Gregorian melodies often begin and end musical phrases at the same points.
Related chants in the Gregorian and Old Roman repertories differ mostly in ornamentation and surface detail.
Old Roman chants are often highly melismatic, with melismas blending into one another and obscuring the underlying melodic structure.
Introits in the Old Roman Mass retained the versus ad repetendum, a repeat of the verse, which had disappeared from the Gregorian chant by the 11th century.
Old Roman Graduals fall into the same centonization families as their Gregorian counterparts, although with variations.
Later, most Old Roman Responsories repeated just a portion of the respond, a practice that was borrowed from the Gregorian convention.