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Consider the scale degree D major. Can I say that the iii (F#-A-C#) and the V (A-C#-E) share the same function because F# and A are distant by a minor third ?

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  • The iii is a suitable substitution for the I as they also overlap, I + iii create a I maj7 in poly chord theory. It is also the relative minor of the V so can be used to replace that.
    – user50691
    Jun 11, 2020 at 17:21

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iii has just as much in common with I as it does with V. Sure there are cases depending on how voices in the harmony are moving iii will act like V and bring you back to I, but not necessarily with the same tension-resolution as a V7. Due to this, while possible in some cases most would look at it more of an extension of the tonic like vi rather than a dominant.

Two other things to add:

All major keys are functionally the same so anything that we talk about function wise is just as valid in D major as it is in Bb major or F# major. The other is while common notes help drive harmony they are not the only thing that determines function. For example a D major 7 has the same common notes as iii does with V, but no one would consider it having the same function as a V chord.

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There are theoretical traditions that make this exact claim, and indeed they extend it to other functions. In short, they say that the function of a given chord (say, V) is also given to the chord built a third lower (iii). And of course, this works in all keys, not just in D major specifically.

Thus tonic (I) function is also present in the vi chord, and subdominant function—which we would today more correctly call "predominant" function—is also present in the ii chord.

This theory thus explains how a iii can occasionally (rarely) function as a dominant, vi can be the goal point of a "deceptive" (or "interrupted" in British English) cadence, and how IV and ii often work together moving towards dominant.

One of the explanations for why this works is because this third relation means that the chords share two notes: I and vi both share scale-degrees 1 and 3, for instance.

But this then presents another problem: if sharing two notes means a chord has the same function, then why does iii share a function with V and not I? (After all, iii and I share two notes as well.) Perhaps iii has both functions? Or, perhaps it's the presence of the leading tone—the main dominant indicator!—and not the tonic pitch that gears it more towards V.

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    "why does iii share a function with V and not I": the question arises from unrealistically expecting a one-to-one function equivalence. But since they are different chords, there has to be some difference. iii can work as a substitute for I when V would sound completely different. It depends on where the harmony is coming from and where you expect it to go. Another example is vii dim, which can work as a V7 or ii. In my ears its role depends on whether I'm expecting a "vi tonic" or "I tonic". If we're in C, then Bdim is like G7/B goint to C, but if Am is tonic then Bdim is more like Dm6/B. Jun 11, 2020 at 20:21

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