The majority of sources of plainsong are manuscripts, and there is evidence of wholesale recopying of the plainsong repertory in various institutions in the fifteenth century. Printed sources of plainsong also appear in the fifteenth century, considerably earlier than they do for polyphonic music. Plainsong resounded in a number of different ways in the fifteenth century. Plainsong melodies were performed as equal notes, or were perhaps organized in repeating rhythmic cells, or sung as if they were mensural, or in a free manner. Some were conceived as cantus fractus. And plainsong could always serve as the basis of polyphonic embellishments of greater and lesser sophistication. Modern scholarship has been increasingly concerned with identifying the exact plainsongs that underlie polyphonic compositions. The operating assumption is that when a composer composed a work based on a plainsong cantus firmus, he chose a version of the melody that was current in the institution where he happened to be working at the time.