Complex sentence: Celtic chant is the liturgical plainchant repertory of the Celtic rite of the Roman Catholic Church performed in Great Britain, Ireland and Brittany, related to but distinct from the Gregorian chant of the Sarum use of the Roman rite which officially supplanted it by the 12th century.
Split into simpler sentences: Celtic chant is the liturgical plainchant repertory of the Celtic rite of the Roman Catholic Church performed in Great Britain, Ireland and Brittany. It is related to, but distinct from the Gregorian chant of the Sarum use of the Roman rite which officially supplanted it by the 12th century.