Notating "enharmonic" chords, e.g. iv7 vs bVI6

Question

I've seen those two chords bVI6 and iv7 used interchangeably for what seems like the same context following I -> IV chord movements in B sections (sometimes continuing to the dominant, sometimes not), for example in the Beatles' song "I Saw Her Standing There" in the bar before the final A section where they "wooo!" I'm wondering what context rules affect which chord to write.

With progressions like that where there's a descending bass walking I-bVII-VI-bVI it's typically over the chords I / I7 / IV / iv7, which makes the most sense to me as it's part of the typical 1-4-5 idiom. I usually hear that minor sixth as the IV shifting minor, as is typically done in B sections when you're descending from root to dominant, where the I7 after the I doesn't change roots to a bVII. But for that song in particular, I've seen more versions with it notated as a flat 6 chord.

Is it all about the gut feeling for which note is emphasized the most to serve as a root, or does some rule related to modes or song structure apply here?

Answer 1

There are no rules, but which one you use is dependent on a concept called "Voice Leading". The basic concept of voice leading is to never use the same voicing of a chord twice, thus adding harmonic color to your music. However the issue of how to notate a chord, neglecting the voicing, I will say from experience, will get you into many musical argument. My mentor once gave me a sheet of paper with the same chord notated differently 7 times and told me to play it, What fallowed was an absolute waste of time.

Edit: whoops looks like I made a bit of a mistake, I first learned about this concept here:

looking into it this is sort of right or at least has a good amount of useful info and I just misinterpreted. What I really should say is that good voice leading never uses the same voicing twice. Either way, sorry bout that.

Answer 2

In popular music they are typically considered interchangeable, so do whatever and likely no one will care.

In classical music, chords should always be named according to their function. Determining a chord's function requires looking at the chords before and after it. In other words, context is everything.

A "rule" of classical music is that dissonances must be resolved. How they resolve is part of their function. Dissonant chord tones (eg 7ths and suspensions) are expected to resolve downwards. (Since added-sixth chords are not a regular feature of classical music, I'd probably treat the 6th as a suspension.)

Put another way, the dissonance is going to be between 2 notes, and so where those 2 notes go in the next chord can clarify the current chord's function. A great example is the difference between a dominant 7th chord and a German augmented 6th chord. They are enharmonically the same notes, but they have drastically different resolutions, and therefore different functions, and therefore different names/spellings.

So, when you don't know how to label a chord, look at how it resolves. Its resolution tells you its function, and its function should help you name it.

For your specific case, iv7 vs bVI6 (assuming C minor):

In the iv7, the dissonance is the 7th, an Eb, where as in the bVI6 the dissonance is the 6th, an F. What chord comes after the chord in question? If you see the Eb resolve to a D (in the same voice), then it's clearly a iv7. On the other hand, if you see the F resolve to an E, then it's probably a bVI6.

All that said, pop music often ignores the rules of classical harmony, so you might not see any resolution at all. In which case, you might want to look at which note is in the bass. There are fewer inverted chords in pop music; which is to say most chords are in root position. So if the bass is playing F, it's a iv7. If the bass is playing Ab, it's a bVI6.

You can read more about the distinction on wikipedia:

Answer 3

There are no rules to do it this way or that way. They are really almost exchangeable. When I remember how I "discovered" the ♭VI chord - before this I used to play IV - iv - and compared with this "new" ♭VI-chord I found this one less boring, more interesting, as the other one was somehow corny. (Today probably both are ;)

Well, it must have been a gut feeling, but of course they don't fit exactly the same to all melodies.

One reason to me learning the Ab chord was probably that I was not able to play this one as a barré chord on the Guitar while the Fm was easier.

Another nice progression - apart yours examples - is to introduce it in a progression like this:

C - E7 - am - A♭

This song inspired me to write half a dozen in this style:

(Those were the days!)

The biggest difference between Fm7 and A♭ will probably be: A♭ with additional ♭7 will become a new function as German 6th chord and in the ear it is very close to this one , while all three chords are often resolved to I64.

After all I think answers to this question will be quite individual and rather based on our personal history of listening than of theory.