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In music notation, is it okay to change the key signature within a measure? Or should I split the meter into two for that?

For example, I have the following four quarter notes in one measure with:

♭ 4/4 A, B, C, D |.

What if I want to change the key signature from ♭ to ♯♯ after B? Should I do it like this:

♭ 2/4 A, B | ♯♯ C, D | ?

Elements In Space
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brilliant
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    You can do exactly what you want - it's your music. But ask yourself - are you actually changing key in the middle of a bar, or could you wait, maybe even longer, until a more opportune moment comes along. just because there are non-diatonic notes involved doesn't mean there's a key change needed. But, if that's what you need to do, just do it as John suggests. – Tim Jul 11 '23 at 20:51

3 Answers3

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It's not common to change key signature in the middle of a bar - my edition of Behind Bars says "change a key signature after a barline", with no qualifications, suggesting that's the usual way to do things.

But, nevertheless, changing the key signature mid-bar is still seen occasionally: for example in Beethoven, Piano Sonata 31: Key signature changing mid-bar in Beethoven's Piano Sonata 31

(which also has a nice example of changing time signature between barlines, as well).

There are also other examples - from Beethoven, in the introduction to the fugue of the Hammerklavier, there are three more key changes between barlines, two of which can be seen below: enter image description here

and in the final movement of Sonata 9:

mid-bar key signature change in Beethoven's piano sonata 9

plus a few clearly metrical examples from other composers - Ravel, Le Tombeau de Couperin: mid-bar key signature change in Ravel's Le Tombeau de Couperin

and one from Busoni, Piano Concerto Op.39 (but there are at least five in that work): first mid-bar key signature change in Busoni's piano concerto

Whether that's the best way to notate your music is a different question, I guess - but I think it's valid, if that's the way you want to go.

Note that according to traditional practice, the old key signature should be explicitly cancelled with natural signs before the new one is written. From my edition of Behind Bars, page 92:

Natural signs cancel a key signature before a new one is stated. The naturals should appear in the order of the key signature they are cancelling.

The following page states that in contemporary practice it's okay to just write the new key signature without cancelling the old one with naturals (unless the new key signature is 'no key signature'). In a search through a bunch of my piano music, it seems like both styles are used.

James Uffindell-Birt
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    Great example by the way; that's a feast of notation. – Elements In Space Jul 11 '23 at 22:32
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    "still seen occasionally": the older the music, the less likely to comply with modern standards. Modern composers also need not comply with modern standards, but whether an established publisher would honor the composer's wishes no doubt depends on the composer's stature and the logic behind the mid-bar key change. It would be possible to re-notate the Beethoven with both the key change and the meter change at a bar line, and it might be easier to read at sight if one did so. Then again, it might not be. This passage seems impossible to understand without a bit of contemplation. – phoog Jul 12 '23 at 07:44
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    Having looked at the score, I see that it's worthwhile to point out that the passage isn't metrical; from the mark "recitativo" to the 12/16 time signature, the bar lines are irregular. In this context the mid-bar changes are less surprising. – phoog Jul 12 '23 at 08:08
  • Yes, I think they're less surprising in a recitativo - my ABRSM edition also puts the key change in the previous 'bar' in the middle of a bar, while most other editions line-break there and add a new barline. There are also three (!) mid-measure key changes during the introduction to the fugue in the Hammerklavier, and at least two of those are similarly 'recitative'-y - the key changes during the fugue itself all happen at barlines. – James Uffindell-Birt Jul 12 '23 at 08:21
  • The manuscript is available on imslp. It's messy, but it looks like Beethoven put bar lines only on line breaks. The first line break in that passage, however, has no bar line, and the third, also a page break, just before the 4-sharp signature, has no bar line at the end of the line but seems to have a faint bar line at the beginning of the subsequent line. The others all have bar lines both at the end of the line and the beginning of the next line. Surely there is much opportunity for interpretation here as to the significance of any given bar line, or lack thereof. – phoog Jul 12 '23 at 08:37
  • Looking at a few editions, it seems like if the typesetter needs to make a line break, they sometimes try to align it with a key change and insert a 'barline' there - but not all editions do, and there are plenty that leave the key-changes mid-measure (especially in the Hammerklavier, and the sonata 9), so I think it's perfectly reasonable to say it's something that's "seen occasionally" in professionally-typeset music. – James Uffindell-Birt Jul 12 '23 at 08:54
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    In artistic fields, you generally need to be a real master to start breaking established rules. The payoff needs to be significant. – Barmar Jul 12 '23 at 16:05
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For some reason I am not seeing some of the symbols you wrote correctly but regardless of that it is completely acceptable to modulate in the middle of a bar. It is best to just use the correct accidentals for the modulated notes in the second half of the bar and then change the key signature staring on the next full bar.

If you really want to show the new key signature exactly when it happens then you can use your idea or two bars of 2/4 but since there really is no change in the time it is not necessary to do it that way. The appropriate accidentals will suffice.

Here it is both ways. You can see that interrupting the flow of the 4/4 just to put a key signature has a bit of an awkward look to it:

enter image description here

John Belzaguy
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Elaine Gould is quite clear about this in Behind Bars — The Definitive Guide to Music Notation. Under the chapter Key Signatures — Key Changes, p. 92, she states unambiguously:

Change a key signature after a barline.

No explanation is given, but I can imagine a few cases where changing key signature mid-measure would be potentially confusing.

For example: If the key signature is changing from C major to G major, the last note of the bar is an F#, and you want to change key signature just before this note. If you place the new key signature mid-measure it would be unclear whether the sharp symbol was a change of key signature or just an accidental.

Elements In Space
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    I find it's easier to read if one uses accidentals in the candidate bar and delays the key change until after the bar. The accidentals give a hint to the reader that something is going to happen. – ttw Jul 11 '23 at 22:54
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    @ttw indeed. I don't know what Elaine Gould has to say about it, but there is also a practice -- at least in vocal parts -- of giving enharmonic equivalents in parentheses, for example if one is changing from E major D flat major one might use this technique to point out that there is no pitch change between G sharp and A flat and that B and D flat are a diminished third apart, so only two half steps. – phoog Jul 12 '23 at 08:25
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    @phoog Gould does mention that in the Vocal Music chapter: Enharmonic spelling, p. 437 – Elements In Space Jul 12 '23 at 08:46