The tension between surface rhythm and the organization of musical time into a regular succession of pulses existed from the beginning of rhythmic notation in the West. Johannes de Garlandia, writing about the rhythmic modes in the mid-thirteenth century, cites a rule (“Unde regula”) that “every note that falls in an odd-numbered position ought to concord with each odd-numbered note” (“omne, quod fit impari, debet concordari omni illi, quod fit in impari”). This stipulation falls well short of constituting a comprehensive theory of musical metre, but it does seem to discourage us from perceiving rhythmic music of this period as consisting of a series of undifferentiated beats. Still, thirteenth-century notation makes no provision for indicating where stress should fall. The transition from modal to mensural notation in the second half of the century did not bring any further clarity to the situation.