My 1990 Gregorian Missal shows a flat at the beginning between the clef sign and the first neume. This might lead you to believe the B is flat throughout the chant. However, my modern notation Liber clearly shows the flat is only for the second pitch of the incipit and subsequently shows a natural sign for the next B. Thus B is natural from there to the end of the chant.
Yes, it's pretty clear, when you look at subsequent lines that do not have the flatted B in the clef signature. Fortunately, any ambiguity from the first line is ameliorated by the absence of any further B pitches in that line.
...and never (I'm prepared to say) in the Proper chants.
I'll admit to re-engraving Propers like 'Tu mandasti' where every ti (or B) is flatted with such a one-flat chant key signature.
My students, who are new to solfege, do not need to be confused by a consistently singing the scale "fa so la te do" in such a chant when it could just as easily (and more helpfully) be "do re mi fa so."
I understand the historical theory behind why this isn't done in the original, but I find changing it in those situations where every ti is flatted to be more helpful for my general musical pedagogical aims.
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