After vertical lines marking chant endings, those delineating between performers are the easiest to classify.
In Barnard MS 1, The Common of the Apostles begins with the chant “Tradent enim vos in conciliis.” At the end of the first three words, there is a (very faint!) double line, or two vertical lines placed closely together. Double lines occur with some frequency in Barnard MS 1, clarifying transitions between music to be sung by the soloist (the cantor) and the full assembled choir. Usually, as in this example, this line appears after the second or third word; less commonly, a double line is placed after the fourth word or after the first word.
Sometimes, the scribe denotes the opposite switch as well. On many occasions, a double line is placed at the end of the chant to show a transition from full choir to cantor. Here, the double line after “Quorum” serves both to mark the end of the chant as well as notate a change of performer; the double line after “Fuerunt” denotes the change back.
However, the occurrence of two complementary double lines together (one at the end of a chant and the other shortly after the beginning of the following chant) is hardly standardized. As in many cases, the second double line is replaced by a single line–here, the vertical line after “estis.”
In this last example, the single vertical line performs the same function as a set of double lines–marking a switch of performer from cantor to choir or vice versa. There is no indication, however, that in Barnard MS 1, double lines have any other purpose. In the feasts I studied, double lines never occur in the middle of chants; even without a clear pairing, they always show a change of performer.
