 The modern concept of the conventual Mass is laid out in the General Instruction of the Roman Missal.Before the reform of the Missale by Pope Paul VI, concelebration was not permitted, and so each priest in a monastery or other house celebrated his own Mass privately, and then participated "in choir" in a conventual Mass celebrated by one of the priests of the house.The system of feasts and requirements of liturgical celebrations were also far more complicated than they are today.