What is the X in X-mas?
plainchant melody and text originally sung responsorially-i.e., by alternating choir and soloist or soloists. Responsorial singing of the psalms was adopted into early Christian worship from Jewish liturgical practice. Most frequently the congregation sang a short refrain, such as Amen or Alleluia, between psalm verses sung by a cantor. As medieval plainchant developed, more elaborate refrains (R) were sung by a choir alternating with soloists singing psalm verses (V), producing a musical form R V1 R V2R. The responsory, or refrain, was frequently abbreviated on its repetition. Its text usually related to the meaning of the feast day or the content of the psalm. Only a few such chants survive in this long form, which is now normally curtailed.