Altogether, an average Mass would comprise about 20 versets. Church contracts that survive from that time show that the organist played not only at every Mass, but also during various other offices and ceremonies. An organist may have been required to play at as many as 400 services a year, which means that much—if not all—of the music was improvised, and the surviving masses can be regarded as models for such improvisations. Musically, the majority of the organ Masses were modelled after the "Missa cunctipotens genitor" (Mass IV in the modern collection), which contained chant melodies. However, composers differed on how they treated these melodies in their works. The 1662 ceremonial specified that the original melodies had to be clearly audible in certain versets, and in cases when "Missa cunctipotens" was used, organists generally complied.