The mensural system of rhythmic notation used in the fifteenth century, musica mensurabilis, was largely inherited from earlier centuries. This chapter focuses on aspects of the mensural system that led to the most interesting features of fifteenth-century rhythmic style. It describes current scholarly arguments about aspects of fifteenth-century notation. Students who learned mensural notation in the fifteenth century most likely did so from a textbook that was a century old, the Libellus cantus mensurabilis attributed to Johannes de Muris. For the modern musician reading fifteenth-century notation, imperfect notes are very close to modern note shapes, since they are the origin of binary system. Johannes Tinctoris's highly polemical remarks about improper use of proportion signs in Proportionale musices are both entertaining and enlightening for the modern reader. By the early fifteenth century, successive diminution of a tenor line was generally not written out but signaled by a verbal canon.