I was told once that I should use thirds up or down to write a harmony.
I tried this but often I have to use both 3 semitones 3rds and
4 semitones 3rds to write the same harmony.
Is there a rule to know when to use a 3 semitones 3rd instead of a 4 semitone one, or vice versa?
Or is it done by personal choice?