I have started to break down this broad question to specific, smaller questions.
Say, I have a song revolving around a theme in E minor. Can I somehow convincingly build it up to change to a theme in, for example, A minor?
An example: you want to express the feeling when someone is buried in deep, moody thoughts for a while, then recollects herself/himself, and finds a solution. So the first part is in a slow, moody theme in E minor, and you want to fluidly arrive in an uptempo A minor theme and then end the song in that theme.
The question is not as easy as it seems (at least for me). One jazzist friend said, that "It's simple! Just throw in a II-V-I chord progression in the new key!".
Unfortunately it does not work when it does not fit the logic and patterns of the song, as we unconsciously understand those larger patterns. So in a jazz song, where chords are flying in at high speed, this solution can work, but when you have a repeated (though varied) theme, you just can't throw in some chords from nowhere.
When I tried to create such transition, I succeeded in that the change felt OK, but you had a feeling that the song would revert to its original key => it was not a convincing change.
Probably these can participate in the answer, but I'm not sure:
- Should I go "up" to new key (i.e. bass of the first theme is E3 and I move it up to A4) or "down" (bass from E3 to A2)? Which one feels more like an "arriving" to the new theme?
- Should I "end" the first theme and "start" the new one, or try to make a seamless transition from one theme to other (like writing a transition section with parts and bits from both sections)?
If you answer, please provide convincing examples from real world songs. Thanks!
For me, the chords/scales part is only about 10% of the problem. Let's narrow the question to more rigid styles, with clear, well-defined sections (exclude jazz, because it's too playful in this manner): say pop, or pre-modern classical era, even rock, even cosmic black metal! Or just one singer, without any instruments.
Say, you play one theme for a minute.
- How long should be the transition between the two clear themes? (Exclude the obvious sudden version). It must be proportional to how many bars was the first theme. Or not?
- What's your strategy if the rhythm of the dominant melodies of the two sections differ?
Narrowing down again, I'm interested in a bunch of ideas for a "seamless" transition, like "X did that in this composition [link], and he is seamlessly changing from A section to B, and that's what X did for that.".
Also an idea for anyone else, struggling with this issue:
- Try to temporarily transpose section B to the key of section A. If you can't make a logical, convincing transition without changing key, I don't believe it is possible with it.