The other day, I looked up the chord sequence for a song I like. It turns out it goes like this:

G, F, C, D, Em, Am, D, G

Looking at the rest of the chords in the song, this is clearly and unambiguously in the key of G major. And, really, the chords aren't anything unusual or surprising.

But look again at the chords I listed. What is that F major chord doing there? In the key of G major, that is an illegal chord. So why is it here? More to the point, why doesn't it sound wrong? Until I looked this up, I had no idea anything unusual was going on. I certainly didn't hear the song and think "oh man, that second chord sounds weird!" What arcane musical theory did they use to achieve this?

(Incidentally, the verse is the only place where this F major chord shows up. Every single other chord in the song is in the key of G major and follows the typical sorts of patterns you'd expect for that key.)

  • It's probably a "mixolydian moment," but to get the most accurate answer, please specify the song, or ideally add an image of the sheet music. Music theory analysis is based on context; it's like asking "What does the word 'head' mean"; it can be a body part, a leader, a verb meaning to lead, a reference to the top of something like an article, etc. Jan 6, 2023 at 16:10
  • @AndyBonner - doing that will bring the question dangerously close to violating the site rules, won't it?
    – Tim
    Jan 6, 2023 at 16:42
  • Illegal chord? Thanks for starting my day with a chuckle. Music would be much less interesting if we weren’t allowed to use chords that are non-diatonic. Here’s a nice article about that specific chord: icce.rug.nl/~soundscapes/VOLUME22/… Jan 6, 2023 at 16:48

2 Answers 2


There have been so many questions similar to this, I wonder who's spreading the 'theory' that all chords must be diatonic. 'Cos it ain't necessarily so.

Any piece in any key can (and often does) contain any chord just about anywhere.

However - that diatonic theory is probably taught to keep beginners on the straight and narrow. The wider theory says that chords can be used from the parallel key. In other words, your F is found in key G minor, the parallel key of G major. Another theory is that your piece is actually written, not in G major, but G Mixolydian - which also contains that F chord. That may then negate the use of a D chord - but you're happy with that, anyway!

  • Beginner theory is often not mentioning things like mixolydian. It mostly focuses on a major and minor scale and how to harmonize them. Blues and country theory might mentioning mixolydianin in the beginning. Pop music or gregorian chant are often not the focus in music theory (although we have music theory for pop and gregorian). People should study the theory of the music the want to play or sing. Jan 6, 2023 at 17:22
  • @Tim first you release us from the restriction of being diatonic. Then you double back and require us to be diatonic in a closely related key! Can't you just completely accept non-diatonic notes/chords?
    – Laurence
    Jan 6, 2023 at 21:21
  • @Laurence - I can, but can everyone else?
    – Tim
    Jan 6, 2023 at 21:50

We can take (at least) two approaches to this.

  1. Who told you that only diatonic chords were 'correct'? And, come to that, who told you that non-diatonic chords had to be justified by being diatonic in some other, connected, key? Not so. 'Diatonic' is a framework, not a restriction. And I'd argue that the concept of 'borrowing' is largely unnecessary.

  2. ♭VII is so commonplace in popular music that it can be considered 'honorary diatonic'. Likewise iv and ♭III.

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