A system devised in the 13th century which governs the rhythmic relationships in music. This system lead to the modern use of time signatures in music. It has to do with the divisions of time in the following manner:
These relationships can be either 3:1 (perfect) or 2:1 (imperfect). By adjusting these relationships and mixing them among each other, many different divisions of time (just like modern time signatures) were achieved.