The Radif of Persian music is the traditional repertoire of the classical music of Iran that forms the essence of Persian musical culture. More than 250 melodic units, called gushe, are arranged into cycles, with an underlying modal layer providing the backdrop against which a variety of melodic motifs are set. Although the main performance practice of Persian traditional music unfolds through improvisation according to the mood of the performer and in response to the audience, musicians spend years learning to master the radif as a set of musical tools for their performances and compositions.