Embellishing the Liturgy

There was a monk named Notker of St. Gall who began writing hymns to add onto the end of the Alleluia part of the mass.  If I understand it right, he did this as a way of helping him remember the melodies of other parts of the mass.  Eventually, the practice of composing new melodies to be added onto the end of the Alleluia became quite popular and common.  These new parts were called sequences.  This was the first step in breaking away from the rigid musical structure that had existed for centuries and in composers being allowed to exercise their creativity.  Soon, writing sequences was not enough, and people began composing entirely new pieces, put to new text.  These new pieces were usually performed right before the official services started (like a prelude) and were called tropes.  Some tropes were even performed like a little play, in that they were being acted out, as well as sung.  The important thing about all of this is that it eventually led to more and more sophisticated compositions, the rise of polyphony, and eventually modern day harmony.