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I looked through similar questions and didn't really see anything that answered a broader question so I guess I'll ask it.

As I write music, my chord progressions tend to go through a lot of different keys, and when I start to transcribe them, I run into the problem of what key signature to go with. For example, say a progression modulates through the keys of C minor, G minor, D major, and F minor (they are all distinct). Would it be best to go with the key signature of the first key, and add accidentals as needed, or the key signature that has the most accidentals in common? D major uses sharps while the rest use flats, how would I accommodate for that? And in general, for any progression similar to this, what is a good way to know what signature to use?

Also, what would be an example song that does this so that I could use it for reference?

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    One option is to simply not use a key signature at all.
    – Lazy
    Jun 10 at 18:28
  • I think this depends on the intended use. When I'm sightreading in an orchestra or chamber music rehearsal, frequent key changes throw me off somewhat. If it's a piece that's being worked up, and I'm not sightreading, then the key changes can be somewhat helpful.) On the other hand, too much black on the page (too many accidentals) can be annoying. But then think of fake books -- apparently jazz musicians (I am not one) rely strongly on the name of the chord. But then jazz musicians are a lot more flexible in their reading and understanding of tonality and harmony than most string players. Jun 10 at 18:56

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Polytonality with a single key signature

In a truly polytonal piece — meaning a piece in two or more keys simultaneously — it's really the composer's choice. For example, in George Gershwin's "Impromptu in Two Keys", the key signature is three flats (for Eb major — the "principal" key of the piece) even though the melody is largely in D major. Accidentals are used for the melody, and its key is clear.

Parts with independent key signatures

In some cases a composer will use different key signatures to reflect the key of each part. One example of using different key signatures can be found in Ligeti's "Etude #7" for Piano. That piece, in which each hand is dedicated to a different whole-tone scale, uses a key signature of Eb-Db for the right hand and Bb-Ab-Gb for the left hand. Etude #12 has a key signature of no sharps/flats for the right hand and five flats for the left.

"Hybrid" key signatures

As suggested in the question, a hybrid key signature can be used, though this more often reflect a piece written in a mode or "manufactured" scale. Bartók does this, for example, in the piece addressed in Unconventional key signature: sharps on F & G only?

Lots of modulation, but not polytonality

The piece described in the question would not be considered polytonal as the various keys are not occurring simultaneously but, rather, sequentially. In that case, the key signature is most commonly determined by the key acting as "home base" — typically found at the end of the piece, presuming the ending isn't intended as ambiguous, but could also be the key that generally dominates the piece overall.

Ambiguous key

In a truly ambiguous situation, it's really up to the composer whether to use a key signature at all and, if so, which one to use. The guidance would just be to use a key signature that either best reflects the composer's intention or one that is most convenient for reading the score.

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  • I feel like not using a key signature at all is an option that could be added to this answer, as mentioned in a comment. Jun 10 at 21:05
  • @ToddWilcox Thanks. I've updated the final paragraph to more clearly express that option.
    – Aaron
    Jun 10 at 21:13
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Frequent Key Modulation

Whether you use one key signature or many key signatures for a passage that frequently changes keys often depends on genre. I've read at least one Music Stack Exchange answer that says that movie soundtrack music often uses no key signature at all throughout the piece.

In comparison, Baroque and Classical-era music often sticks to one key signature in passages and entire pieces that frequently change keys, especially development sections of sonata-allegros. See the sheet music of the prelude of this edition of J.S. Bach's English Suite No. 3 in G Minor, BWV 808 (the key signature is 2 flats throughout, the change to D minor does not get a key signature change, there is also a long passage that modulates several times) and the sheet music of part of the development section of the 1st movement of this edition of Beethoven's "Appassionata" Sonata (the section notated with no key signature has portions in E major, E minor, C minor, and A flat major) for two examples. Key signature changes often align with major section changes in Classical- and Romantic-era music - see the Rondo Alla Turca sheet music in this edition of Mozart's Piano Sonata No. 11 in A Major (key changes to A major (from A minor) consistently get key signature changes) and the scherzo-and-trio 3rd movement of this sheet music of Schubert's "Death and the Maiden" string quartet (the trio, which is in D major instead of the scherzo's D minor, gets the appropriate key signature change) for two examples.

Modern music is more likely to actually change key signatures with key changes - for example, more official sheet music of Winter Games by David Foster actually gives the piece's G major section a 1-sharp key signature, while leaving the rest, which is in F major, with a 1-flat key signature. But don't always count on that happening! The sheet music of the concert band piece Shadow Rituals by Michael Markowski gets away with using only a concert 3-flat key signature throughout the entire piece regardless of all the key changes it goes through (e.g. this analysis of Shadow Rituals says that "Shadow Rituals’ primary tonal center is E-flat Phrygian, with periods of B-flat (major and minor) and C minor. The work ends in A-flat and D-flat Phrygian" in Page 27).

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