Medieval music consists of songs and pieces from an era of Western music, including liturgical music (also known as sacred) used for the church, and secular music, non-religious music. Medieval music includes solely vocal music, such as Gregorian chant and choral music (music for a group of singers), solely instrumental music, and music that uses both voices and instruments (typically with the instruments accompanying the voices). During the Medieval period the foundation was laid for the notational and theoretical practices that would shape Western music into the norms that developed during the common-practice era. The most obvious of these is the development of a comprehensive music notational system which enabled composers to write out their songs and pieces on parchment or paper. Prior to the development of musical notation, songs and pieces had to be learned "by ear", from one person who knew a song to another person. The development of music notation made it easier to disseminate songs and musical pieces to a larger geographic area. However the theoretical advances, particularly in regard to rhythm—the timing of notes—and polyphony—using multiple, interweaving melodies at the same time—are equally important to the development of Western music.