The long fifteenth century was a decisive period for song as an artistic form: songs began to attain equal footing with and distinguish themselves from masses and motets. While masses and motets stand as discrete artistic products whose origins can often be traced to liturgical song, composed songs are open to the limitless field of universal practice. As important and decisive as polyphony was in fifteenth-century song, its textural fabric is also characterized by profound instability. By the time of Guillaume de Machaut, the three-voice chanson had become a normative model, even if the number of voices could be augmented by a triplum above or beneath the cantus. Deschamps is one of the first theorists of poetry to comprehensively treat the so-called formes fixes, which underpinned chansons from Machaut until around 1500, with parallels in other Romance song traditions. The immense growth in chanson production in the fifteenth century led to greater diversity and differentiation of subjects.