So I was looking at the chords of a simple song in G major scale. The only 3 chords it's using are G, C and F, all major. i don't understand from where you get the Fmaj? if it was F#min, that makes sense to me. Is this as a result of mixing G and F scales together?
1In addition to the two great answers, note that you were probably expecting F# diminished, not F# minor: F# A C is a diminished triad, whereas F# A C# would be the minor triad, and there is no C# in G major.– Richard ♦May 27, 2017 at 0:06
See diatonic modes and specifically the Mixolydian mode. Just like how minor and major scales are built up of half steps and whole steps in a specific pattern, there are many other scales that use different patterns of steps. G Mixolydian contains the notes for G, C, and F major. You can think of the Mixolydian scale as a major scale but instead of a major 7th you lower it to a minor 7th. There are multiple ways to think about the theoretical basis of modes and this is a useful one.– user35889May 27, 2017 at 9:38
1I had the exact same question except I worded it differently to cover any key that uses a flatted 7th scale degree as a major chord as a substitute for the regular scale degree as a diminished chord. I was thinking of just such a song in the key of G with an F major in it when I posed this question which has some great answers that also answer your question (music.stackexchange.com/q/29817/16897)– Rockin CowboyMay 27, 2017 at 22:30
The song has a tonal base of G major. This is not the same as 'using only the G major scale'. As you have discovered! No need for any special justification. Just know that the bVII chord (In the key of G, that's F major) is frequently used to add a bit of colour.
“Key of G” means only in the simplest sense that all your notes come from the G Ionian scale. The F note is borrowed from the nearby Mixolydian mode. This is very common in blues, pop and many folk styles.
Often this sort of thing is explained with parallel keys. Songs in G major can use the notes, therefore the chords, from G minor. In Gm (natural) there are F A and C, producing the F chord. So it's sort of in the family. That apart, in the key of C, the three prominent chords just happen to be C, F and G. The difference here is that most songs in G will have the F# note (usually accompanied by a D chord, sometimes Bm or F#o). Using an F note in the melody sometimes has G7 under it, but the F chord fits sonically, if not in the basic theory.
If you want to look at it from a different perspective, think modally. G Mixolydian contains the same notes as C major, so the F chord could be construed to come from that.
1Or just think from the position that being 'in G major' doesn't restrict you to the notes of G major scale. It saves inventing a lot of complicated 'reasons' for non-diatonic chords.– LaurenceMay 27, 2017 at 23:01
@LaurencePayne but that makes it seem too arbitrary. Most non-diatonic chords would sound much more “out”, for the reason of not being easily constructible from first principles (without a 12-edo grid). Whereas the ♭Ⅶ is really right next door on the circle of fifths. May 27, 2017 at 23:37
And includes two diatonic notes. Which may be more relevant, if we're not always basing our music on ii, V, I patterns.– LaurenceMay 29, 2017 at 0:23
"Including two diatonic notes" is, of course, a consequence of being "close in the circle of fifths". My preference is to think modally: an F major chord in G major is a bit of mixolydian flavor. Why invoke G minor or C major? At least for me, an F major chord in G major doesn't suggest or imply G minor or C major- but maybe I've just played too much mixolydian.... May 29, 2017 at 22:18
@ScottWallace - in fact, thinking about it, it might be the ultimate theory - consider everything modally, all modes are usable, therefore any note out of the twelve, or any combinations thereof, are acceptable to use. Need a reason - think modally! Still concerned? Think chromatically...– TimMay 30, 2017 at 7:11
This is simply called borrowing. In this example, it is most likely borrowing from the key of C major (C, F and G are very commonly used chords in this key) as C is the 4th of G.
One could also consider the use of F major as a way to bring in the dominant 7th of G - dominant 7th of G is F whereas the major 7th is F#.