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Relative modes share the same note.

I was told the key signature could mean either major or minor. But can it not also mean the other five modes like Phrygian, Locrian, etc.?

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3 Answers 3

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Yes, the major/minor key signatures can represent any of the diatonic modes. However, it's not uncommon to use the key signature for the "nearest" major or minor key and then use accidentals to adjust for the mode.

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    To spell it out, by "nearest" key this means Lydian = major + raised 4, Mixolydian = major + lowered 7, Dorian = minor + raised 6, Phyrgian = minor + lowered 2. (I say "raised" and "lowered" and not "sharp" and "flat" because in some cases the necessary adjustment is a natural - for example in F Lydian you'd change B flat to B natural.) Feb 26 at 14:58
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There are two ways of writing key signatures of modes. One is to use the key signature of the parent key, the other is to use the traditional key signature based on the root note of the mode, as if it was a key in its own right.

Using the first method, it will give the reader the expectation of the parent key, just like any key sig. would. In fact, (let's take one sharp) that would tell G major or E minor. We only know (or suspect) which it actually is by looking for the last note/chord, or searching for the D♯ which would tend towards E minor.

Or, as Phoog points out, since Phrygian (B in this case) is minor, a key sig. of Bm could be used - 2 sharps would tidy up things, and only leave all the C♯ notes to be naturalised. The Phrygian mode of G is B Phrygian. Using the key sig. of B would be pretty cumbersome - 5 sharps, 4 of which may well need cancelling each time they appeared, making the music dense, and not so easy to read, I think.

So, it would depend on the writer, who decides whether the second method is in fact harder or easier to read (and write!). But rather like the original major key sig., which only accurately reflects notes from the scale, the first method is more favourable.

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    A more likely choice of key signature for Phrygian is that of the parallel minor, not the parallel major. That is, you might use the key of B minor and just cancel the C sharps with accidentals.
    – phoog
    Feb 26 at 8:50
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    @phoog - very good point, it's been incorporated into the answer. Many thanks!
    – Tim
    Feb 26 at 10:37
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Yes.

What you are really asking is when presented with a key signature how do I know the tonic and mode of the music?

For example, let's say the key signature is one sharp. You've probably been told that is the key signature for one sharp is either G major or E minor. But how do you know which? Without going into too much detail, determining whether the music is major or minor mode is a type of quick harmonic analysis.

Usually that harmonic analysis is done with a quick reading of the starting and ending of the music. You look for the tonic which will be something like the final tone of the melody or the root of the final chord. If that tonic were G, the music is in G major, and if the tonic were E the music is in E minor.

You would follow the same basic analysis steps to determine whether the music was in a mode other than major or minor. For example, in general terms, if the music ended with a tonic of A (again, assuming a key signature of one sharp) the mode would be A dorian.

Whether the music is in phrygian can be a bit complicated depending on which modal style the music uses. "Modal" can be used to describe music in jazz, rock, folk, and early "classical" styles of Medieval and Renaissance music. Each of those styles can treat phrygian mode differently.

Again, let's assume a key signature of one sharp. The phrygian tonic for that key signature will be B. Using just the basic diatonic tones of the key signature the step above the tonic will be a C natural and the tonic chord will be a B minor triad. A potential source of confusion for you could be phrygian music in Medieval or Renaissance styles where the second scale tone may sometimes take a sharp and the final triad is very often a major triad. So, in our example key signature of one sharp, that would mean seeing some C sharps and the final chord being B major. Without getting into details, you can think of those things as characteristic chromatic inflections of phrygian mode in Medieval and Renaissance style music.

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