Do you avoid parallel octaves/5ths when writing parallel supporting Melodies? For example when harmonizing a vocal melody I have a plugin that by default adds a 3rd and 5th underneath. Should I change it to a 6th and 3rd? I read you shouldn’t have a parallel 6 3 chords more then 3 times in a row. So should I automate it to use the 3rd and 5th once in a while. Also. Let’s say I have a chord underneath and the 6 3 chords that are following the melody creating non chord tones with the harmony. Is that okay? Kinda like bichordal stuff. I’m confused with parallel supporting melodies.
There are some "rules" or guidelines about this in classical harmony theory. First parallel movement is generally considered less desirable than counterpoint or oblique movement but this is really a matter of preference. However, when it comes to having 5ths and octaves without a note in between one reason to avoid it could be resonance. These are considered "perfect" intervals. They contain a large number of matching overtones or harmonics. As such the intensity of sound will build higher than a harmony with 3rds even if the players or singers are not increasing their individual volume. For this reason it's avoided, however rules were meany to be broken and they have been. It is not uncommon in modern music to play parallel 5ths and octaves.