How did 13th-century composers use rhythmic modes?

In 13th-century music, composers used rhythmic modes as a system of theoretical abstractions to codify and organize rhythmic patterns in polyphony, particularly within works like organum, clausula, conductus, and the motet.12

Composers typically employed six distinct patterns, which were analogous to poetic meters such as the trochee, iamb, and dactyl. These rhythms were not indicated by individual note shapes but were instead conveyed through standardized groupings of ligatures—compound symbols that represented specific sequences of long and short values.1

In forms like the descant clausula, all voice parts were set using these repeated rhythmic patterns. However, as musical demands grew more complex, the limitations of this system became apparent. By the mid-13th century, composers began developing individual symbols for different time values, eventually transitioning toward mensural notation, which provided a more flexible and precise method for notating rhythm.3145