I was wondering, how can you fit powerchords behind vocals to fit in harmony?
I understand that octaves and minor thirds and fifths work, but being just that, seems a tad limited. is there an easy explanation when I want a series of notes from the vocals over say, one powerchord being sustained? And what about riffs behind the vocal notes? How do I get them to fit nicely in harmony?
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Your questions are unclear to me.

Let me try and rephrase your question and let me know if it's what you are looking for.

I have a vocal melody and now want to put chords to these notes, how to I go about it? What if I want to use mainly power chords?

What key is it in, where does the melody fall at the start of each bar

Lets say, Key of C major (epic fun)

1st bar, the melody starts on C, second bar, it falls on F, third bar it falls on A and so on

Chords you could use would be Cmaj, Fmaj and Amin

OR if you're into using power chords, C5, F5 and A5 - but please for the love of all things holy, do more than just power chords, at least have another instrument fill out the tonality - like playing arpeggios on a piano etc. etc.
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If your question is harmony, no harmony is outlawed or anything, it's all about learning what the sound is and adapting it and making the "best" use of it to your ears and experimentations .

Like Tom above says, I suggest trying first to make the melody fit into obvious chord tones such 1 3 5 ,

C F A melody

Possiblie harmonies in C key sig

Am Dm F
F Bm7b5 Em

First as tonic, second row is melody as third above, then 5th above.

After you LEARN and HEAR these sounds, you go for extensions on chords, then non chord tones, then figure out how to use your library of sounds for your spirit.
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