Notation had developed far enough to notate melody, but there was still no system for notating rhythm. A mid-13th-century treatise, "De Mensurabili Musica", explains a set of six rhythmic modes that were in use at the time, although it is not clear how they were formed. These rhythmic modes were all in triple time and rather limited rhythm in chant to six different repeating patterns. This was a flaw seen by German music theorist Franco of Cologne and summarised as part of his treatise "Ars cantus mensurabilis" (the art of measured chant, or mensural notation). He suggested that individual notes could have their own rhythms represented by the shape of the note. Not until the 14th century did something like the present system of fixed note lengths arise. The use of regular measures (bars) became commonplace by the end of the 17th century.