I'm counting measures and confirming measure numbers in an opera score, and I've come across a discrepancy. I wonder what this means about conventions for numbering measures in opera scores in general. Do some measures not get numbers, or did I uncover a mistake?
The situation: it's a piano-vocal opera score. Page 67 has three systems. The first system is measure 620, and is one measure long. The second system is not labelled with a measure number. But it has four measures, then a double-bar-line, then a new key signature and time signature at the end of the system. The third system has its first measure labelled as measure 624.
Since system 1 starts with measure 620, and is one measure long, then system 2 should start with measure 621. However, it is not labelled. Since system 2 has 4 complete measures, system 3 should start at measure 621 + 4 = 625. But the score says 624.
It looks like the notation at the end of system 2, introducing a new key signature and time signature, gets no measure number. I'm can understand that.
What's going on musically? Systems one and 2 are the end of a duet between Violetta and Alfredo. The singers end on the second-last measure of System 2. The music plays out through the last full measure of system 2. It is a 3/8 measure which ends a phrase. The first two pulses are a chord. The last pulse is a 1/8 rest with a fermata. This measure is a conclusion and a pause, before the music resumes with a new key and time signature and tempo too, as it turns out. In System 3, Gastone enters and starts a new bit of music.
Is there a convention that ending measures like this get no measure number? That would explain why the next measure is labelled 624 instead of 625. Or, is this likely an editorial mistake?
Similarly, if there is a pickup measure at the start of a piece, does that measure usually get a number?
The score, if you are wondering, is Verdi's La traviata, piano vocal score based on the critical edition, published by Ricordi and the University of Chicago Press, c2001. ISBN 8875926743. Since it's a critical edition, I think it's more likely that the numbering is as intended, and I don't understand the rules. But even Ricordi can make mistakes, I suppose.
It would be helpful if I could post a image. Unfortunately the score is under copyright, and I don't want to violate that. I could locate the corresponding measures in one of the public-domain editions at IMSLP. You can look in your own Traviata score: Act I, end of Violetta-Alfredo duet with, "ah! dimenticarmi allor" (system 2), followed by Gastone entering with, "Ebben? Che davo fate?" (system 3).
[Update 1: added more detail to "what's going on musically?" paragraph, and explanation for no image.]
[Update 2: on looking more closely at the end of system 1, I see there is no bar line at the end of the system. Instead there are pairs of turn symbols on each set of staff lines. Together with the absence of a measure number at the start of system 2, this support Pat Muchmore's interpretation that all of system 1 and the start of system 2 form a single measure, which has many more beats than the time signature calls for.]