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Messiaen's Prelude for Organ (no opus number, discovered posthumously in 1997, composed probably circa 1929) is rife with ambiguous accidentals. Some can be figured out by examining similar passages, but two cases are particularly egregious:

In bar 59, alto, are the unmarked G noteheads flat or natural?
In bar 61, alto, is the last G sharp or natural?

Asked another way: at those chromatically altered unisons in soprano and alto, which accidental applies to which voice? Is there a rule for that? If we knew the alto's accidental, we could then let it continue for the later alto notes.

(Were the note heads (or the staves!) separate, with an accidental immediately preceding each, the confusion would vanish.)

Explanations from the musical text, from a facsimile edition if such exists, or from rules of notation outweigh reports that so-and-so played it in such-and-such a way.

Messiaen, Prélude pour Orgue, bars 59 and 61, ed. Leduc

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    Isn't bar 61 just a typo? I mean what does a sharp immediately followed by a natural mean at all? If the composer intended both notes to be played, it is very poor typography. – Alexander Pacha Sep 30 at 15:26
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    It's not a typo (a slip of the finger), it's sloppy notation (a slip of the brain). One accidental applies to the soprano, the other to the alto. – Camille Goudeseune Sep 30 at 19:29
  • See also this post, and the one it's marked as a duplicate of, and the one that that is marked as a duplicate of. – Arthur Oct 1 at 14:05
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If you make the assumption that the voices don't cross, and that the accidentals apply to the voices individually, things seem to fall into place.

In the second half of bar 59, if the lower voice is G flat and the upper is G natural, the lower voice has the pattern

Eb F Eb F Gb

Eb Gb Eb Gb G

which has some logic to it.

Similarly in bar 61, if the upper voice is G sharp and the lower is G natural, the final G should be a natural, which repeats the D#-G tritone at the start of the bar.

Ideally, these details should be interpreted with reference to all the rest of the score, but we don't have that available. Messiaen's music is usually highly structured, so these passages are unlikely to be just "free atonal improvisation."

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    D#-G isn't a tritone. – Camille Goudeseune Sep 29 at 20:37
  • I'm accepting this answer because "no voice crossings" was the key to sleuthing out the rest of it. But I'm posting my own answer with all the icky details. – Camille Goudeseune Sep 29 at 21:18
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  1. Here's the argument for bar 59's altered unison's soprano being G natural (and thus alto G flat).

    • In the whole piece, voices never cross. At least not more than a semitone in cases like these, so it would be statistically odd to have only those as voice crossings. Simpler to say that none were intended at all.

    • In bars 58-60, soprano traces out G natural rising to D, filled in variously.

    • Were bar 59 alto's last note G natural, then it would have been tied to the G natural immediately across the barline. Every other repeated notehead in the whole piece (except for one case in the solemn slow introduction), even across voices and across staves, is scrupulously tied, e.g. bar 61's A-A. This happens at least a hundred times. So alto must be G flat, and hence soprano must be G natural.

  2. Beyond no voice crossings, the only argument for bar 61's altered unison's alto being G natural is that it avoids making the bar's final chord a prominent open fifth, which would be a purposelessly startling consonance in this densely chromatic texture. There's no parallel passage to compare this bar to.


Gardner Read's "Music Notation," 2nd ed., pp. 73-74, covers how to stem and mark altered unisons. But it says nothing about the case at hand. All the examples use multiple note heads, to prevent ambiguity about which accidental applies to which voice. By forbidding a single notehead (even two-stemmed) with multiple accidentals, this avoids inventing a morass of rules such as "the first accidental applies to the upper voice."

So the published notation is rubbish. Even if the manuscript used this notation and the editor was too reverential to improve it, at least a footnote to that effect would have been justified!

  • 1
    Adding to confusion/rubbish factor is that the combined natural + flat is the same notation used to indicate that a prior double-flat is canceled and replaced with a single flat. – wrschneider Sep 30 at 18:58
  • Yes, even though such cancelling (similarly, cancelling a double flat with a double natural) is becoming archaic, the confusion would have been so easy to avoid. – Camille Goudeseune Sep 30 at 19:27

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