Complex sentence: Old Roman chant is the liturgical plainchant repertory of the Roman rite of the Early Christian Church formerly performed in Rome, closely related to but distinct from the Gregorian chant, which gradually supplanted it between the 11th century and the 13th century AD.
Split into simpler sentences: Old Roman chant is the liturgical plainchant repertory of the Roman rite of the Early Christian Church. It was formerly performed in Rome, closely related to but distinct from the Gregorian chant, which gradually supplanted it between the 11th century and the 13th century AD.