What are the rules of writing a solo for guitars or violins on a keyboard?

Question

What are the harmonic rules that serve as the foundation of a solo? For example, notes of triads serve as the foundation for voices and arpeggios, with an occasional non-chord tone. Harmonic function serves as the foundation for chord progressions.

However, most solos I've come across seem to ignore the rules of chords, note intervals, range, and sometimes even keys. Additionally, the composition of solos seems to change depending on which musical genre you're in.

I don't know which ideas I should start with when writing solos. Other than fiddling around my keyboard and hope that I get lucky with the notes, is there a better alternative?

Edit 1: Aside from obvious ranges of an instrument, are there any limitations when writing for solos?

Also, what happens if I attempt to transpose a song and pushed the notes of the solo out of the instrument's range? How do I rewrite the solo section without changing the underlying "message" of the solo, since I can no longer rely on the inversion techniques for chord-based instruments?

Answer 1

The only rule is: idiomatic writing. You must write a line for an instrument with its own techniques and limitations. For this, you need to understand the language each instrument speaks, so you need to hear a lot of examples of this or that instrument playing in solo, group, different techniques, timbres, effects and so on.

Ignore this and you will end up playing keys, but with a different tone.

Answer 2

notes of triads serve as the foundation for voices and arpeggios, with an occasional non-chord tone

Fellow site user Richard, in his answer here to another recent question, linked to a paper called The melodic-harmonic ‘divorce’ in rock that addresses this point. Some of what is explores is that in a lot of popular music writing, the melody doesn't always seem to have a strong relationship with the chords(though I would still say that melody and chords do tend to reference the same basic tonality, however independently).

Harmonic function serves as the foundation for chord progressions.

In some genres, yes; in others: not at all. Many subgenres of rock tend to avoid the dominant, for example!

However, most solos I've come across seem to ignore the rules of chords, note intervals, range, and sometimes even keys. Additionally, the composition of solos seems to change depending on which musical genre you're in.

Well, 'the rules' are different depending on which musical genre you're in. That's the very reason that different genres of music sound different: they have different forms and functions, audiences have different expectations as to how harmony and melody should work; they may use diffrent instrumentation which affects the way the harmonies are actually percieved, and so on.

I don't know which ideas I should start with when writing solos. Other than fiddling around my keyboard and hope that I get lucky with the notes, is there a better alternative?

As some people have suggested in the comments: learn the musical 'language' of the genre you're interested in writing for. Guitars and keyboards can be found in almost every genre of music, and they do very different things in each; violins aren't quite so ubiquitous, but can still be played in a wide range of styles.

Some genres have some amount of theory already out there to describe them and get you started; other genres, you may have to do some of your own study.