I hope you will excuse this maybe trivial question that I haven't been able to find an answer to, possibly because I don't know how to answer it properly.

In the key of C, we know that the I-IV-V progression is C-F-G. F is the fourth note of the scale and G is the fifth one.

However in the key of, say, G the progression is now G-C-D. From what I understand C should then be higher than G. Yet it seems whenever someone is talking about the I-IV-V progression on the key of G, the C they are using is actually an octave lower, similar to the C of the I-IV-V progression of the key of C!

Why does this happen? Does this mean that you can actually use a chord from any octave in this progression?

  • yes, you got that correct; you can use a chord from any octave (see octave illusion) – Lenny May 9 '17 at 19:42

In music we have the notion of octave equivalence which states that, in various musical contexts, all Cs, no matter what the octave, are equivalent.

So your C in the bass can go up a fourth to F, then down a seventh to G, then down another fifth to C---a C an octave below the original C!---and everything's fine.

A famous music theorist by the name of Heinrich Schenker came up with the notion of the "obligatory register," stating that these octave equivalences are really alterations of a more normative, closer register. His preference for this bassline was for C to go up a fourth to F, up a second to G, and then back down a fifth to the starting C.

With that said, octave equivalence can sometimes lose its luster when it's a melodic line that's being played. Imagine playing:


and then the final


is three octaves higher than it should be. Yes, it "resolves," but not quite in the way we wanted, wouldn't you agree?

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  • The melodic point about B to C is probably especially true because it's the leading tone moving by step to the tonic. A move like F to G down a seventh seems more acceptable. – Michael Curtis May 9 '17 at 21:00
  • And the octave equivalence is not an inherent property of C right? It should hold for any note. – user40189 May 10 '17 at 10:20
  • @Aarkham Yep, octave equivalence applies to all pitches, not just C. – Richard May 10 '17 at 18:53

Playing a I IV V with all chords in the root position can sound disjointed. If you play chord I in second inversion, chord IV in first inversion and chord V in root position or first inversion, everything will be contained within one octave for a smoother 'stepping' (as opposed to 'leaping') effect.

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When someone says "a C chord" or "a G chord", they are referring to any such chord; not one in a specific register or with a specific inversion.

The same applies when saying "a IV chord" or "a V chord". The only additonal restriction added beyond the above is the specific function within the key. A C major chord can be played in any context, but a IV chord is only a C major chord in the context of G Major.

A progression is just a movement between chords, so it also does not specify any additional constraints on the chords. If a progression should be purely ascending, that should be specified! If it should ibe an exact set of notes, then they should be written out as sheet music, tablature, or some other explicit format.

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