Mensural notation is the musical notation system used for polyphonic European vocal music from the late 13th century until the early 17th century. The term "mensural" refers to the ability of this system to describe precisely measured rhythmic durations in terms of numerical proportions amongst note values. Its modern name is derived from the terminology of medieval theorists, who used terms like musica mensurata ("measured music") or cantus mensurabilis ("measurable song") to refer to the rhythmically defined polyphonic music of their age, as opposed to musica plana or musica choralis, i.e., Gregorian plainchant. Mensural notation was employed principally for compositions in the tradition of vocal polyphony, whereas plainchant retained its own, older system of neume notation throughout the period. Besides these, some solely instrumental music could be written in various forms of instrument-specific tablature notation.
Mensural notation grew out of an earlier, more limited method of notating rhythms in terms of fixed repetitive patterns, the so-called rhythmic modes, which were developed in France around 1200. An early form of mensural notation was first described and codified in the treatise Ars cantus mensurabilis ("The art of measured chant") by Franco of Cologne (c. 1280). A much expanded system allowing for greater rhythmic complexity was introduced in France with the stylistic movement of the Ars nova in the 14th century, while Italian 14th-century music developed its own, somewhat different variant. Around 1400, the French system was adopted across Europe, and became the standard form of notation of the Renaissance music of the 15th and 16th centuries. Over the course of the 17th century, mensural notation gradually evolved into modern measure (or bar) notation.