Why does the iii chord function as a tonic chord if it has a leading tone?

Question

I am learning chord functions in major scales and supposedly the iii chord has a tonic function. however, the chord has a leading tone so shouldn't it be considered a dominant chord?

Answer 1

The idea is from German theory.

I don't know that theory well, but apparently the mediant chord can have both tonic and dominant function. The reason would be the overlapping tones of the tonic (^3 and ^5) and dominant (^5 and ^7) chords.

My way of reconciling the leading tone ^7 being in a non-dominant chord (and also the dominant ^5 is in the mediant) is two-fold:

• the ^3 degree has its basic identity as a member of the tonic chord
• the absence of ^4 - which could provide a dominant seventh - makes the chord less emphatically a dominant.

In other words the real dominant identity is TI & FA (in solfege) or scale degrees ^7 & ^4. It may seem ironic but ^5 isn't so critical a factor, but keep in mind ^5 is in the tonic chord too; clearly not a dominant in that case. TI & RE make a good dominant. You might put it like this: the leading tone TI and the absence of potential tonic tones - for practical purposes FA or RE - make a clear dominant. Then SOL or ^5 provides... I suppose tonal stability.

Having said that I thought the German theory treated the mediant chord as a dominant and the submediant as a tonic. Using the English terminology of the Wikipedia page the relative relations of I/vi and V/iii are somehow more fundamental that the counter-relative relationship of I/iii.

I don't know how that theory is actually applied in analysis. I learned American theory where all seven scale degrees are discrete chords. I'm not sure how a analysis case is made for the mediant being either dominant (Dominantparallele) or tonic (Tonika-Gegenparallele.)

Answer 2

The third is not "tonic", it's called mediant. The reason some people call it tonic (and the vi submediant), is because, just as the vi chord, it moves in a 3rd step from the I chord, making it a very stable movement. Let's take C major key first as an example (C D E F G A B), (and then I will explain the tonal system after).

C major C E G
E minor E G B

From C major to E minor, every note (root 3rd and 5th) moves in 3rd steps.

A minor is A C E
C major is C E G

The same thing as for C major to E minor, but this time going backwards along the scale.

The 3rd degree of a key is called the mediant, the 6th however, is also a 3rd step from the root if you go backward from C to A. This is why it's called the submediant, from E to A is also a 3rd, which makes going from E to A very stable, and this is why mixing I, iii and vi always sounds good despite the permutation.

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Now I will try to explain 2 principles of tonal center:

Every key has a tonal center, which is the tonic.

Moving a 2nd forward and backwards, produces a weak movement that makes you want to move back to the previous note. It doesn't matter if it's the tonic or any other note from the key, moving in 2nd is generally undesirable. UNLESS you want to loop I > ii > I > ii, or in other cases, playing modal chords, which is a different system from the functional harmony.

Moving by 3rds produces a stable movement, it's not too big that it feels like a leap, nor too small that pushes you back to the previous note. In the functional harmony as I explained, this is called mediant.

Both G major and B diminished have 2 notes that are a 2nd degree from the tonal center C:

G major G B D
C major C E G
B dimin B D F

G major is the strongest move to C major, because it not only has two 2nd steps surrounding C (B > C < D) creating a lot of tension, but it also shares a common note which is G, making it perfect for a smooth voice leading.

B diminished has the B and D surrounding C, but it doesn't share a common note with C major. Some people prefer to use B diminished with the 7th degree, that way B and D resolves on C, the F resolves on E, and the 7th (A) resolves on G, making it a bit more pleasurable than hearing just B diminished.

This is why V and viiº are called dominant.

That leaves us with the ii and IV chords.

The IV chord (F A C) shares a note with the I, which is the tonic itself (C). If we voice lead IV > I it should look like this:

F C A becomes C F A
C F A
C E G

C stays the same, F to E will be a 2nd movement, and A to G will also be a 2nd movement, giving it a weak feeling of resolution.

And last, probably the most awkward for me, the ii. Every note from the ii chord moves a 2nd step from the I chord, which naturally pushes you back to the I. But unlike the IV chord, it doesn't share any note to voice lead.

Which is why most chord progressions that uses the ii, actually starts from the ii instead of starting from the I tonic, there is no much you can do with the ii chord, the only place to go apart from the tonic is to move a 3rd step to the IV chord, giving that mediant feel, and then resolves back to I, in jazz, ii V i using 7th chords is the most essential progression, but that's a different topic on using 7ths and voice leading…

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That's it, I have explained basically the whole theory behind functional harmony, lol.