Modes and Mensuration Signs
Unlike the original system of Franco of Cologne, which was geared towards the invariant metric pattern of 6/4 (with inherently triplex longa), later compositions from the 14th-century Ars nova onwards could display a greater variety of basic metric patterns. They can be defined as different combinations of duplex (imperfect) and triplex (perfect) subdivisions on successive hierarchical levels:
| Maximodus | perfectus | imperfectus |
| 1 maxima = 3 longae | 1 maxima = 2 longae | |
| Modus | perfectus | imperfectus |
| 1 longa = 3 breves | 1 longa = 2 breves | |
| Tempus | perfectum | imperfectum |
| 1 brevis = 3 semibreves | 1 brevis = 2 semibreves | |
| Prolatio | maior | minor |
| 1 semibrevis = 3 minimae | 1 semibrevis = 2 minimae |
The perfect modus and maximodus were rare in practice. Of most practical importance were the subdivisions from the brevis downwards (by that time, the semibreves and no longer the breves had taken over the function of the basic counting unit). The four possible combinations of tempus and prolatio could be signaled by a set of mensuration signs at the beginning of a composition: a circle for tempus perfectum, a semicircle for tempus imperfectum, each combined with a dot for prolatio maior, or no dot for prolatio minor. These correspond to modern 9/8, 3/4, 6/8, and 2/4 meters respectively.
| Tempus perfectum | Prolatio maior | 9/8 | |
| Tempus perfectum | Prolatio minor | 3/4 | |
| Tempus imperfectum | Prolatio maior | 6/8 | |
| Tempus imperfectum | Prolatio minor | 2/4 |
Read more about this topic: Mensural Notation, Ligatures
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