Amid lessons and chants that tell the story of the first martyr, the Kyrie Clemens rector aeterne was sung at Mass on the feast of St. Stephen. Although Clemens rector aeterne Mel.102 does not appear in tenth- and eleventh-century sources as often as two most popular Kyries, it is widespread among the early Aquitanian manuscripts. Transposition of entire phrases, such as in Clemens rector aeterne, reveals a sophisticated formal sense: such transposition is more complex than exact repetition, on the one hand, or simple difference, on the other. It provides both the consistency afforded by repetition and the diversity afforded by contrast. The motivic design of Clemens rector aeterne depends upon a dozen or so motives, each just a few notes long; together these account for most of the melody. The motivic structure supports the formal scheme just described, but its importance lies beyond that.