First Inversion Mediant (III6)

I was doing some part writing for a music theory class where I was tonally harmonizing a bass progression into four parts. It was during one of these progressions that I encountered the dominant in the bass. I've been trying to explore my freedoms here, so rather than use a V or i6 chord, I was curious about whether I could use a III6 chord. When I thought about it, it occured to me that the III6 and the V chords are quite similar. The first two notes of both chords are the Dominant and the Leading Tone, the only difference is the change of the supertonic to the mediant. However, I could not find any information on this specific chord, nor any in depth information on the purpose of the supertonic versus the mediant so I'm hoping perhaps one of you might be able to satisfy my curiosity.

How is a III6 chord used in tonal composition?

I see that there are no posts in this topic yet, so I will offer my highschool music theory nerd understanding of it. The iii is more like the I than the V. A lot of people say that iii-I is a retrogression because it's too weak a resolution with the leading tone up to the tonic being the only change. The ii is usually used as a pre-dominant chord. I'm pretty sure iii6 can take you into most predominant chords. I'm not so sure that it can take you into neapolitan chords, though I don't really see why not. I feel like movement slows down when a iii chord is used, especially vi-iii6. My understanding of the mediant chords purpose in traditional harmony is that it acts as an alternate way to go to a pre-dominant chord. Also, it's an easy excuse for a V/vi, and every time I hear one of those I think of Georgia On My Mind (I-V43/vi-vi)

Well, even though iii is somewhat more rarely used, I don't think it requires any special explanations. You can use it in various ways as other first-inversion chords, also as passing or neighbouring chord. Sharing two common tones with the tonic and the dominant, III is an intermediate chord between these two functions and can take both of them, though not as expressive as the primary chords, of course (also, III is not as close to the tonic as VI). So, yes, you can use iii6 instead of V (IV-iii6-I, for example), in which case it could be seen as V with a sixth. By playing the fifth scale degree, the bass makes the dominant side of the chord more prominent.

Also, note that in minor, there is some tendency to III, which is the tonic of the relative major. Using III a lot, especially together with VII, can sound destabilising and also can be used for modulation.

Down by 3rd is not really a retrogressive root motion. Up by 3rd is retrogression.

It should be noted that in function theory (the harmonical analysis theorie most often used in Germany, specifically), iii is called the dominant parallel or dominant substitution, as VI is the tonic parallel and ii the subdominant parallel. So in this theory, ii, iii, and VI are mainly secondary functions that can take the place of the primary I, IV and V. And indeed, this often is how they are used. Using iii, or more specifically iii6 in place of V is relatively common, and the only analytical question is whether you want to describe it as an inversion of iii, or whether you prefer describing it as a V with a 6th suspension (which often doesn't resolve), or even (if the tonic follows), as a V with anticipated third of the tonic. It doesn't matter too much in the end.

The III6 in minor can also be an interesting bridge between variant minor and major keys. In C-(harmonic)-minor, the chord G-B-Eb for instance can be re-interpreted as a G-B-D#, representing the V of the variant major key with raised fifth, as leading tone to the tonic's third. Such enharmonic games are of course extremely common for all augmented and diminished chords.

Yeah, it is actually taught the same way (Riemannian) here, in Bulgaria, and when it comes to analysis, usually the symbols T, T_VI, S, S_II, D, etc. are used (I suppose it is the same way in Switzerland, Austria?).

Thanks everyone, that's exactly what I was looking for. You've satisfied my curiosity, I'm going to fool around with some III6 chords now. :)

Yeah, it is actually taught the same way (Riemannian) here, in Bulgaria, and when it comes to analysis, usually the symbols T, T_VI, S, S_II, D, etc. are used (I suppose it is the same way in Switzerland, Austria?).

I don't know about Austria, but in Switzerland we mostly analyse by scale degrees (although, with the relatively high number of professors around who are either German or have studied in Germany, function theory is relatively widely known as well). It's definitely useful to be aware of different methods of analysis, since all of them have certain downsides and other aspects where they truly shine.

Thanks, it is good to know how it is in Switzerland. Personally, I tend to prefer the functional approach, it often makes things clearer and also has a psychoacoustical justification. Even when I use the Roman numeral system, I still look for functions in the background of my mind.

By the way, it's interesting to note that in jazz harmony, the tonic category consists of the three chords I, vi and iii. Classically, the 'tonic' function of iii comes mostly when it prolongs tonic harmony.