Rooted in musical language itself, the transformative impulse emanates from a drive to explore the semiotic potential of written song. Before about 1430, composers had written out series of intended mensurations in sometimes lengthy verbal instructions; thereafter they began to indicate mensural reinterpretation by using a different mensuration sign for each iteration. Mensuration was more often signaled intrinsically, through coloration, the grouping of note shapes, and dots of division and perfection, rather than a dedicated mensuration sign. Notes rely on metasigns to activate their meaning. A simple change of metasign has the potential to recast the pitch or rhythm of every note it governs. The aesthetics of notational fixity fueled the transformative impulse, evincing a fifteenth-century interest in things sounding other than what they seem. Canons specified the meaning of red notation, indicated mensural reinterpretation, and clarified individual signs.