During the medieval period the foundation was laid for the music notation and music theory practices that would shape Western music into the norms that developed during the Common Practice period of shared music writing practices which encompassed the Baroque era (1600–1750), Classical era (1750–1820) and Romantic era (1800–1910). The most significant of these is the development of a comprehensive music notational system which enabled composers to write out their song melodies and instrumental 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. This greatly limited how many people could be taught new music and how wide music could spread to other regions or countries. The development of music notation made it easier to disseminate (spread) songs and musical pieces to a larger number of people and to a wider 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.