This chapter introduces mensural notation and shows how changes of practice affect musical style during the Renaissance, mirroring the approach in the previous chapters in the domain of pitch. Here again, two crucial changes are observable, roughly contemporary with those observed in Chapter 5: the first is the gradual abandonment of major prolation (c. 1440) as the predominant mensuration in favour of perfect tempus, and the alternation of tempus perfectum and tempus imperfectum (with minor prolation) as a structural feature in larger-scale works; the second is the adoption of tempus imperfectum as the standard mensural sign during the final quarter of the fifteenth century, after which the notational subtleties associated with the system of ‘four prolations’ gradually fell out of use. But this chapter also demonstrates the great elegance of the mensural system, showing its flexibility and economical presentation of situations of considerable rhythmic intricacy, which has an aesthetic quality all of its own. In many cases, the conception of the individual work is grounded as much in its notation as in the sounding result.