I'm composing a song that is in E♭ phrygian which can also be regarded as A♭ aeolian. Do I note this in the key E♭ minor (6 flats) or Aflat; minor (7 flats)?

In general: Is the key oriented on the base note or the number of flats/sharps actually played?

3 Answers 3


Eb minor is simply not Eb phrygian. These are different modes altogether. If you notated it in Eb aeolian = Gb maj, you would need an accidental to get you to Phrygian.

So, is the song really completely on the Phrygian mode or aeolian with occasional flat 2? If it is Eb phrygian it would make sense to write it in Ab minor (Cb Maj, oy).

In general the "key" is not the base note of the mode, but the number of flats and sharps.

  • 1
    What does oy mean in this context? Commented Dec 22, 2018 at 2:26
  • 1
    Like, Oy Vey. Hebrew-Yiddish for woe, as in woe is me. Cb is quite a choice of key.
    – user50691
    Commented Dec 22, 2018 at 3:23

Modes are based on major keys ( and minor, but that's not the issue here).

Just as every major key has its own key signature, which essentially tells which will be the diatonic notes for the piece, there is a relative minor which contains the same notes, thus with the same key signature. Move to notes from not the Aeolian but harmonic or melodic minors, and accidentals become necessary.

All the modes of a certain parent key will, by definition, contain those same notes as the parent key. Thus, it's important to use that same key signature for any mode from it.

Eb Phrygian has its parent key as Cb. An awful key to write or read, and to my mind, totally unnecessary. So, if the piece is in Eb Phrygian, the key signature will be that of Cb major. Any notes which do not match those diatonics from it will obviously need accidentals, but the whole point is that at the beginning of the piece there will be a statement "these notes are those which generally will be those used hereinafter." Obviously every note is a flattened one, including C itself (being Cb but landing on a white key on piano).

Yes, Eb Phrygian is a minor key - every Phrygian is a minor key - but just because it's based on Eb does not mean it's anything to do with Eb minor, which contains a different pool of notes. Base everything on the parent major, and it's (slightly) simpler.

ggcg sums it up here - 'oy'... Begs the question - why that mode (from that parent key!)?


Use the key signature that matches the sharps or flats you have in the scale you are using as a basis for the piece. Any other notes that will be altered from the key signature should use accidentals in the score when those notes are used. Eb Phyrgian would require a flat for every note, so use 7 flats in the key signature.

They key signatures are there for practical purposes, to simplify marking which notes are sharped or flatted. The "root note" is not as much a consideration. We tend to think of key signatures as relating to major and minor keys, but that is mostly because of historical development and the fact that the major and minor scales are the most commonly used. However, in the end, the only question is "which notes are sharped or flatted?" In some modern classical music, mixed key signatures that belong to no key are used because they announce that, for example, a Bb and an F# are used throughout the piece.

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