Jim, I'm not talking about "rules" that much.
I wasn't referring specifically to you with that comment, but more in general.
As to me it's just useful to know from which material chords are likely to be built when I have a melody for instance with a minor third and a major sixth (i.e. Dorian). And yes, I do apply this to the music.
Fair enough. Different tools for different crafts
persons. My own usual methods are less formal. Almost certainly there's an element of what you mention, not as a formal rule that I've learned from someone else, but as something that over time I've learned to feel
... both from listening to the music of others and through personal experimentation. In fact, on the English concertina some of it is "in my fingers", and if I want to try something different I need to make a conscious effort.
Assume the melody has an E. What simple chords (triads constructed from diatonic scales) contain an E? There are E major, E minor, A major and minor, C major, and C# minor.
Well, if the key signature is one sharp, then E major, A major, and C# minor are out, since all contain a C#, which is not part of the scale. Experience tells me that if the tune seems to be in E minor (Aeolian), Em is the most common chord, then Am, and C the least likely of the three. If the tune seems to be in G (major), my order would be C, then Em, then Am.
Lacking further guidance, I could try all three and decide which I like best... against that particular note at that particular point in that particular tune. But there's more to it than that. Usually the flow of the melody -- the surrounding notes -- will direct me toward a particular one of those chords, not necessarily the "most common". And that's simply a matter of my taste, based on years of hearing music of many kinds and noticing what I liked and what I didn't like. Now maybe that coincides with certain prescriptions regarding "modal" music, or maybe not. I don't know, and I don't seem to need to know.
There's even more to it than that. I sometimes use different chords or harmonies1 for a given note on different times through a tune. In particular, in accompanying a song I might use a major chord in a "happy" verse and a minor chord at the same place in a "sad" verse. Or I might decide for some reason or other to use a chord not from "the list", which would give a very different feeling.
1 I don't believe that all harmonies are "chords" in the usual sense. A simple drone or a single second voice, for instance, doesn't necessarily constitute a sequence of "chords"
If the rules help you, that's great, though I believe that it helps to keep them simple and few. But I also suspect that as you gain experience you'll want more and more to experiment with "breaking" the rules, as you say you did with Apley House
. I think that's good, maybe even important.