Body of singers with more than one voice to a part. For many centuries, church choirs sang only plainsong (seeGregorian chant). The relative complexity of early polyphony required solo voices rather than choral performance, but by the 15th century polyphony was being performed chorally. The growth of the secular choir (or chorus) coincided with the beginnings of opera. An oratorio choir is part of a different tradition, which stems from the augmented church choirs used to provide choral portions of a given oratorio, whether performed in or out of church.