11 There are many statements of the fathers and ecclesiastical writers testifying that after freedom and peace had been restored to the Church the psalms and hymns of liturgical worship were in almost daily use. Moreover, new forms of sacred chant were gradually created and new types of songs were invented. These were developed more and more by the choir schools attached to cathedrals and other important churches, especially by the School of Singers in Rome.
Source: Musicae Sacrae (Vatican.va)