I am sure that any of you who have had cause to work with the New Mozart Edition of the Marriage of Figaro have noticed a curious anomaly in the part of the Countess. At certain moments in the "new" edition, she exchanges parts with Susanna. In the famous trio with Almaviva, she is given the high lines that go up to high "C", while Susanna has the lower part!
This has never made sense to me, and whenever I directed this excerpt, we always went with the Schirmer edition distribution of parts, which seemed to respect the Countess' vocal comfort in a consistent way all through the opera. No Countess that I know has ever appreciated the schizophrenic nature of the vocal writing in the new edition, where the arias have one tessitura for the Countess (lower), and the ensembles another (higher).
It turns out there is a good reason to trust Mr. Schirmer. Alan Tyson, the English musicologist, in his first volume of Mozart Studies, has a very illuminating article on this subject. The New Mozart Edition was drawn on the original manuscript, or "handschrift" whenever possible. In the Mozart manuscripts, however, there are major discrepancies between the "handschriften" and the "abschriften" which were contemporary copies made directly from the manuscript. It seems that in one at least of the "abschriften" Mozart himself altered the Countess' part to the lower line.
Apparently, Mozart himself made some of these changes to the autograph ("handschrift") for the first part of the first act, and then stopped. The end of the opera, in the autograph, is consistent with the Countess having the lower tessitura, and was probably written out after the decision had been made in rehearsal to give the lower lines to the Countess. It certainly looks like Mozart just didn't get around to changing all the parts for the Countess in the original manuscript.
The first edition, which follows the distribution of parts taken up by the later Schirmer edition, has the same consistency in the way the Countess is written. If you think about it, it makes total sense that Mozart may have adjusted the parts in rehearsal, and then written them himself into the abschrift (which functioned as the conductor's score), while the original manuscript stayed at home, uncorrected. Both Entführung and Finta follow the convention of giving the highest parts in the ensembles to the prima donna. The Countess, however, may have been a different type of voice than Constanze or Sandrina, and the high tessitura of the ensembles in Figaro as originally planned, may not have been suitable for her voice.
Sometimes, if it doesn't make sense, it is just plain wrong. In musical editions,"urtext" isn't always better.