ABSTRACT
Although the preserved copies of the Magnus liber organi originated well after the time of the composition of the music which they contain, they inevitably must be regarded as our principal key to the history of the origin and development of this celebrated collection of two-voice organa. There is one group of Alleluias which does include a pair of compositions comparable, in the extent and directness of their interrelationship, to the O 2-O 7 compositions: Adorabo for the Purification BMV and Posui adiutorium (M 51) for the Common of Bishop Confessors. Nevertheless, there still remains a question as to what is the proper or preferred reading. A reading with tenor in duplex longs and a third-mode duplum is perhaps the most satisfactory, but a sixth-mode duplum with longs in the tenor is also possible, while still another possibility is a duplum in first mode with extensio, with duplex longs in the tenor.
