De Mensurabili Musica (concerning measured music) is a musical treatise from the early 13th century (medieval period, c. 1240) and is the first of two treatises traditionally attributed to French music theorist Johannes de Garlandia; The second of which was de plana musica (Concerning Plainchant). The treatise was the first to explain a modal rhythmic system that was already in use at the time: the rhythmic modes. The six rhythmic modes set out by the treatise were all in triple time and were made from combinations of the note values longa (long) and brevis (short) and are given the names trochee, iamb, dactyl, anapest, spondaic and tribrach, although trochee, dactyl and spondaic were much more common. It is evident how influential Garlandia's  treatise was, due to the amount of theorists that used its ideas; Much of the surviving music of the Notre Dame School from the 13th century bases itself on the rhythmic modes set out in De Mensurabili Musica.
^Christensen, Thomas. The Cambridge History of Western Music Theory (Cambridge: Cambridge University Press, 2002), p. 628
^Taruskin, Richard. The Oxford History of western Music - I (Music from the earliest notations to the 16th Century) (Oxford: Oxford University Press, 2010) p196/7
^There has been recent scholarly debate on whether Johannes de Garlandia actually wrote the treatise. Some music historians believe that he was simply editor of the treatises. More information can be found on his Wikipedia page.