   The lower part, the tenor, moves slowly as the upper part, the duplum, is much more active.  The texture of this selection moves from the melismatic organum heard at the beginning to interludes of unison singing.  Notice how the texture moves back and forth between these two textures.  The organ notes are in unison with the tenor part; instruments, when used, were played in unison with the tenor part.  The rhythmic activity of the upper part is based on three notes to a beat.  This is representative of the new development of measured rhythm typical of the school of Notre Dame.  The tenor part is taken from an earlier chant (cantus firmus) and slowed to a drone.  The upper part takes on the melodic activity reminiscent of monophonic chant.