Key signature for writing in modes other than major and minor?

Question

I have trouble deciding what accidentals to put in the key signature for pieces in Dorian, Phrygian, Lydian or Mixolydian modes. There seem to be trade-offs associated with each choice. For example, when writing in G Mixolydian, setting an empty key signature makes it look like C major or A minor. Having the F# in the key signature forces me to put accidentals all over the piece to mark F naturals. I prefer to use (in this example) the key signature of G major and natural signs on every F, especially when writing for choir. Singers with varying reading skills will often unconsciously assume that if the piece has G as the tonic, then it is in G major. The natural sign on every F will help them remember there is something special about it. Is there any kind of convention to follow?

Answer 1

Hi, modes follow the same key signature of their initiated keys. C Major is the scale, D Dorian or G Mixolydian are the modes coming form the C Major Scale. If you write a tune that uses the G Mixolydian, no key signature is fine. Example is Miles Davis's "So What" is in D Dorian, has no key signatures. If we improvise C major or A minor scales, all notes sound great. What determines the modes are the chord progression as much as the scales.

Answer 2

All modal scales share the same accidentals as a corresponding major scale. For example, if you start on the second note of a scale (Dorian mode) it will have the exact same accidentals as the major scale starting a whole step above your original first note. The only difference is the primary tonal center, or lowest most important note.

This sounds confusing but the easiest way to visualize it is on piano with the C scale. The seven white keys of the C scale create seven different modes - Ionian or major with C on bottom, Dorian starting on D, Phrygian starting on E, Lydian on F, Mixolydian on G, Aeolian or Minor on A, and Locrian on B.

Similarly, the 7 notes making a D major scale also create 7 modes - Ionian with D on bottom, Dorian with E on bottom, Phrygian on F#, Lydian on G, Mixolydian on A, Aeolian on B, and Locrian on C#.

This pattern always applies.

So if your key signature looks like E major, it really could be E major, F# dorian, G# Phrygian, A Lydian, B Mixolydian, C# Aeolian, or D# Locrian based on which note you're assigning the most harmonic importance to and making your place of resolution.