The Gradual, like the Alleluia and Tract, is one of the responsorial chants of the Mass. Responsorial chants derive from early Christian traditions of singing choral refrains called "responds" between psalm verses. According to the Catholic Encyclopedia, it (and the associated Alleluia or Tract) is the oldest of the chants of the Proper of the Mass, and, in contrast to the Introit, Offertory, and Communion, the only one that was not sung to accompany some other liturgical action, historically a procession. Until about the fifth century, it included singing a whole psalm. They were sung in the form of a "psalmus responsorius", i.e. the whole text was chanted by a reader appointed for this purpose. For some time before Pope Gregory I, to sing these psalms was a privilege of deacons at Rome, a privilege he suppressed in 595. The people answered each clause or verse with an acclamation. This apparently dates back to the synagogue tradition, and can even be seen in the structure of some Psalms (such as 136|135). Originally, there was a psalm sung between each reading, of which in the fifth century there were three (Prophets, Epistle, and Gospel). When the Old Testament reading was later dropped, the other two psalms became the Gradual and Alleluia, ordinarily sung one after another, until the 1970 Missal restored the three readings on Sundays and Solemnities.