I'm getting my head around song key theory and wondered if any of you guys who know more than me have tips on how to work out the key of a song if it happens to have chords that feature in more than one key and the tonal centre isn't very obvious either.
For example, we know C major has C, A minor and G, but so does the key of G also.
Thanks!
Using your example, if the chords are common to both C major and G major, you need to next look at the scale which the melody (or melodies) use. Is the F in the melody an F natural (suggesting it is the 4th note in C major) or F# (suggesting it is the 7th note in G Major). If you were to sub in a dominant 7th chord, would it sound more at home as the V7 of C major (ie a G7 chord) or as the V7 of G major (ie D7)? You can do a number of analyses like this, to try to work out purely with theory which is most likely. Another thing to keep in mind is that it might be a modal melody/sequence.
If no simple analyses work, you could have an argument for either key and spark a rather interesting debate, giving both arguments and discussing which you find most convincing based on the pros/cons. Does the song feel more like the melody resolves to a C or a G as the tonic note? In a song like Dreams by Fleetwood Mac, the only two chords are F major and G major, yet the melody suggests that the song feels like it is in C major even though the song doesn't technically feature that chord anywhere (as far as I remember at least). Maybe the chord sequence you are analysing doesn't feature the I chord at all.
At its heart, musical theory analysis is very subjective. There are commonly followed rules and opinions, but these can always be challenged. As long as you can convincingly justify yourself, you will have a leg to stand on regardless of what you might suggest as the "correct" analysis.
If we were analysing a Mozart minuet, this would be easy. The melody would be full of nice tidy leading notes (the F#'s that would give us a pretty good clue your example was in G rather than C). And, in any case, you'd be reading it from a printed copy with full notation including a key signature!
But today's pop and rock music often isn't that clear-cut. And even if it was in G, 'bluesy' flattened 7ths are common. Very often there isn't a melody as much as some rather ill-defined shouting. If the tonal centre isn't obvious, LET the key be ambiguous.
