Basics of Key Signatures on the Guitar
Generally, major keyed songs on the guitar center on the 1st degree of the major scale, while minor keyed songs center on the 6th degree. However, you can also center music on one of the other major scale degrees. As a result, you can’t assume that a major key is always Ionian or that a minor key is always Aeolian.
How to look past the key signature to figure out a song’s mode
Here’s where things get tricky. Although the major scale has multiple modes, musicians generally think of and notate music as being in only the relative major and relative minor, even when another mode is being used.
So songs in the major modes (Ionian, Lydian, and Mixolydian) are all treated as if they were plain major, or in Ionian mode, while songs in the minor modes (Dorian, Phrygian, and Aeolian) are all treated as if they were natural minor, or in Aeolian mode.
Music publishers generally disregard the mode and write everything as if it were in a plain major or natural minor key, going off of the tonic chord. For example, if a piece of music centers on a G chord, it’s notated with a key signature reflecting the G major scale even if it’s really G Lydian or G Mixolydian.
If a piece of music centers on an Em chord, it’s notated with the signature E natural minor even if it’s E Dorian or E Phrygian. Then any accidentals (sharps, flats, or natural signs) are used for notes that fall outside of the key signature. As a result, you receive no initial instruction that the music is based in a scale other than the scale reflected in the key signature.
Take the song Seven Bridges Road by the Eagles, for example. The primary chord progression is in D Mixolydian mode. The notes and chords are from the G major scale (G-A-B-C-D-E-Fs), and the 5th scale degree, D, functions as the tonic. Because the tonic chord is D, music publishers notate the song as if it were in a plain D major key signature, which includes two sharps, Fs and Cs.
Then every time a C-natural note occurs in the music, both in the melody and in the chords, they specially mark it with a natural sign (n) to cue you not to use the Cs note reflected in the key signature here.
If this were written with a key signature for G major, you wouldn’t need any accidentals. But, alas, things are never that easy.
Publishers often use the same technique when a piece of music is in Lydian; they write it as if it were plain major and then rely on accidentals to make any changes. For example, C Lydian, which is drawn from the G major scale, is written with a key signature of C, implying the plain C major scale. A sharp sign appears each time an Fs occurs throughout the music.
The same thing happens in minor keys, too. Take, for example, Oye Como Va by Santana. This song centers on an Am chord and is said to be in the key of A minor. However, saying A minor implies A natural minor, the relative minor of C major. That’s incorrect. A natural minor features an F-natural and produces a Dm chord.
But this song features Fss and D major chords. The parent major scale is really G major. Nevertheless, you usually see this song marked with a key signature of A minor with sharp signs next to all the F notes used in the score as seen here.
Note: If this were written with a key signature for G major, you wouldn’t need any accidentals. You can expect to see the same technique used with Phrygian; it’s notated as natural minor and then corrected with accidentals.
One reason that using the relative major and minor keys is standard among musicians is that music often draws from more than one scale. Modal interchange is a common composition technique that mixes different parallel scales (scales that each draw from a different parent major scale yet center on the same pitch). For example, G major, G Lydian (D major scale), and G Mixolydian (C major scale) are parallel scales.