Changes in meter notated by ratios or symbols that reduce or increase the relation of the durations of the notes. The system of proportions is incredibly complex, but the practicing musician need only become familiar with a few in order to perform the great majority of music. However, the availability of proportions made Proportional Canons possible, ie the creation of polyphonic music from a single line by applying both time-relation and pitch-relation changes to produce many parts. Apel shows many of these, which range in complexity from a Josquin 2-part canon to Pierre de la Rue's fuga quatuor vocum ex unica in which one part produces four, each at a different pitch (starting together on G, d, g, and d'), and proceeds at a different rate, so that only one voice actually uses the whole part. Ending places are indicated by signum congruentiae. Also, and according to some theorists, most often, proportions were used to make tenor parts, which would be based on a cantus firmis and changed in each repeat with a new time signature.