Affinity...

long discussion...

"Affinities" are relationships between chords. In the classical tonal system, the most used affinities are those of the fifth, which are established between chords whose roots are a fifth apart. From this arise the three main functions of the tonal system—tonic, dominant, and subdominant—as well as modulation procedures based on the circle of fifths.

In reality, there is another "foundational" affinity of the tonal system: the third affinity that exists between the triad on the tonic and the triad on the tonic of the relative major or minor mode. The relationship between a C major chord and an A minor chord is called a third affinity. Based on this model, Romantic harmony built a gigantic number of possible affinities, always based on the hypothetical link between a chord and the chord at a distance of a third (whether it be major or minor matters little). Not being a composer, I cannot tell you if there are actual rules of usage, advice...

Continuing the discussion on affinities as chordal reflections of the tensions within the tonal system, the leading-tone affinity is a sort of "second episode" of the television series "secondary dominants." So, in reality, it is not an affinity different from that of the fifth; it is just a more specific application of it. A secondary dominant is an external chord that, however, creates tension and resolves onto a chord "proper" to the key. For example, a MAJOR chord on the second degree of a major scale that resolves onto the dominant triad. Well, if we are talking about external chords that resolve, we can speak not only of dominant chords of the dominant but also of leading-tone chords of the dominant. So, IV#dim - V, for example. It is a succession very similar to that created by augmented sixth chords, but in this case, the chord that creates tension is a diminished chord. In Romantic harmony, however, these leading-tone relationships are much more complex. They also occur between major and minor chords, which succeed one another "only" thanks to contrapuntal movements of leading-tone-to-tonic resolution. Which, in this case, is a deceptive resolution. In essence, some complex chromatic successions of chords are built on the leading-tone-to-tonic tension, i.e., on the ascending semitone.

Take it as an introduction; I am certainly not the most suitable person to conclude such a discussion...

Try to enter the world of third affinities (my teacher called them harmonic transitions) by sitting at the piano. You play a C major chord and reason like this:

every component can assume another role (the root becoming the third or fifth, the third becoming the root or fifth, the fifth becoming the root or third). This way, relationships that are not traditionally related will emerge.

For example, from the C major triad:

R->3 : Ab major

3->R : E major

3->5 : A major

5->3 : Eb major.

These are combinations already present in Lasso and Gesualdo, just to name two.

Then there is the 3rd of C major which becomes the 3rd of C# minor.

But if you start from a minor chord, things get more interesting.

From the C minor triad:

R->3 : A minor (C minor - A minor, sounds a lot like Harry Potter, right?)

3->R : Eb minor (this one and the following also sound like Harry Potter, but above all like Incantesimo, that sort of soap opera on Rai1)

3->5 : Ab minor

5->3 : E minor.

And then there is the 3rd of C minor which becomes the 3rd of B major..

Correlations that you find in abundance in film music from at least the last 50 years.

From this basis, then, if you also reason with sevenths, even more beautiful ones emerge.

Thank you. I know I'm arriving late, but when I asked I hadn't quite understood your explanations... not that it's any better now, but after about a year some procedures are a bit more streamlined....