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Can someone please help me make sense of why some flat symbols in the key signature are in parenthesis?

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    I'd really appreciate a clearing-out key signature for the Bars 3-4 of your exercise, which are labelled as being in A minor. Reading your sheet music as-is, I otherwise thought the key signature of Bars 1-2 also applied to them.
    – Dekkadeci
    Sep 21 at 11:45
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    Wow, in 60+ years of reading piano music from long ago through today, I've never seen this! :) Sep 22 at 22:14
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A little-known fact is that the historical basis of minor tonality is the Dorian mode. Consequently, much 18th-century tonal music is written in a key signature that seems to lack one flat sign. This is especially common for chorales, since the tunes are frequently from the 16th or even 15th century, when they were unambiguously Dorian.

The parentheses here indicate that the editor has changed the key signature to align with modern expectations. Many editions make such changes without informing the reader.

In minor keys, of course, the sixth scale degree is occasionally raised, which gives rise to the ascending melodic minor scale. In minor keys notated with a Dorian key signature, the sixth scale degree is instead frequently lowered by a half step. In fact, this tendency goes back to the dawn of staff notation. Many medieval melodies in Dorian mode feature B-flat very prominently.

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  • For the sake of consistency there should also be a flat sign in a parenthesis in front of the Es in such an edition. Sep 21 at 11:50
  • @LarsPeterSchultz - The key signatures in the picture are treated almost consistently - the E flat in the key signature of the G minor section similarly has parentheses, and the D major section uses an A major key signature (but uses G naturals), but the A minor section has no key signature change, bizarrely enough.
    – Dekkadeci
    Sep 21 at 12:04
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    @AndyBonner the symbol is only redundant under the modern understanding of its meaning. In those days, key signature symbols operated, as accidentals still do, only on a single octave.
    – phoog
    Sep 21 at 13:59
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    Your very last sentence - which Dorian mode? They can't all feature Bb !
    – Tim
    Sep 22 at 8:44
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    @Tim in those days there was just one Dorian mode with the final on D and the dominant on A.
    – phoog
    Sep 22 at 10:21
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In addition to phoog's statement that Baroque-era music was often written with one fewer flat in the key signature than we'd expect given the tonic (e.g. J.S. Bach's Toccata and Fugue in D minor, BWV 538 "Dorian"), note that no A's or A flats are used in that C minor cadential phrase that uses the key signature with the A flat in parentheses. The editor has put that A flat in the key signature in parentheses because s/he could get away with it - it's not being used.

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    I know there’s a B, but it has a natural sign in front of it. So to me it seems like this theory would only make sense if the flat for B also had parentheses. Sep 21 at 12:13
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    I recently learned that the other Toccata and Fugue was actually written with the same key signature, i.e., for D Dorian with no sharps or flats. Why it was seen as useful to identify one of them as "Dorian" when they had the same key signature is still a mystery to me.
    – phoog
    Sep 21 at 12:18
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    @ToddWilcox: I have seen this use though. Since this answer is here I don't need to write it.
    – Joshua
    Sep 21 at 21:24
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    @phoog The origin of these antiquated key signatures is correct, but their use continued long after the point where it was justified. Bach's 'Dorian' toccata is not based on any chorale, and it isn't Dorian in any way - all the Bs are in fact changed to B flats right from the start. Even Haydn's early sonatas contain such pseudo-Dorian signatures which no longer make any sense at all. Sep 21 at 21:48
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    @KilianFoth the very idea that the use of such a key signature requires "justification" is anachronistic. The assertion about B flats and B naturals in Bach's Dorian toccata is furthermore incorrect. For example, there is a single B flat in measure 2, but there are four B naturals in measure 3.
    – phoog
    Sep 21 at 23:57

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