When scales don’t help much
As long as the chords used in a piece only contain notes from the scale for the chosen key, the melody can stick to the scale without any clashes with the underlying chords. As an example, if you were composing a piece in C major, you could use chords such as C, Dm, Em, F, G, Am, Bdim, as well as sevenths such as Dm7, G7, Am7, Bdim7, without ever leaving the scale of C major. However, there is no reason why music should stick rigidly to the confines of a particular scale and, thankfully, most composers are happy to go where their imagination takes them.
The example that springs to mind for me is a song called So Don’t Sigh, which relies heavily on a chord sequence of C, Ab, Gb, A. If you look at the notes within those chords, you can see that the melody will have to undertake some contortions:
C major: C E G
Ab major: Ab C Eb
Gb major: Gb Bb Db
A major: A C# E
Here is the song, with the vocal line replaced by an alto sax:
The tables below show the notes of the first two lines of the melody with the chord changes shown above in red.
C | Eb | Gb | A | |||||||
G3 | C4 | G3 | Eb3 | F3 | Gb3 | Ab3 | A3 | E3 | A3 | B3 |
C | Eb | Gb | A | C | ||||||||
C4 | C4 | Bb3 | G3 | A3 | Bb3 | C | C#4 | A3 | G3 | E3 | D3 | E3 |
In this way, you’re really using four major scales in one song
And here’s another example, called Sequis. It follows a basic sequence of C Ab F Ab, C Ab F Ab, G Ab G Ab G Ab G Ab Bb, and that takes the melody on a bit of a dance:
Instruments on Sequis:
Lead guitar: Fender Stratocaster through Guitar Rig
Acoustic guitar: Sequis from Orchestral Tools
Lead synth: Arhkis from Orchestral Tools
Pad: Analog Strings from Output
Bass: Substance from Output
Percussion: Damage 2 from Heavyocity