Hi ijh. As you get more and more into the structure of music, you'll begin to recognize these deviations from the diatonic, like the F chord in the key of G. In many of the lessons I post here, I show examples of others, such as turning the usual minors into majors, or the opposite, turning some (usually the IV chord) of the majors into minors. It really is strictly up to the person composing the tune what chords are used, and more often than not, the modern composers of pop music don't know anything about those related chords ... they just experiment away and settle on chords that sound good together. Naturally, those 7 chords that arise naturally out of the major scale sound good, but other do as well, such as the F in the key of G.
You can easily file these away mentally using the Roman numeral method. That way you can recognize them in other tunes and use them if you want to write your own tunes. The F in G would be bVII, so you can quickly see that it's a major chord (capital) built on the flat 7th degree of the scale, in this case F. Once you start thinking along these lines, you'll see similarities everywhere, and you'll also be able to simply hear those chords and know what they are without even thinking about it.
The main thing to remember though is that 'outside' chords can and do come into play all the time, but if 4 or 5 are from a certain key and 1 isn't, go with the majority. And, while knowing the key is very important to know, it's more important to know the 'Chord of the Moment' ... to my way of thinking, anyway. I know of many tunes where the key is not awfully apparent, but they're great tunes, the Rolling Stones' 'Brown Sugar', for example. The 'verse' section of that tune is C F and Bb, which looks to be the key of F in a V - I - IV sequence, but it's not; the chorus brings in a G7 which is an obvious V chord of C, so in fact it's in C and that Bb chord is the chord you mention: a bVII.
|