if you have the melody and everything in your head how do you figure out a good chord progression to fit the song?
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Find what key that it's in and choose from the selection of chord that belong to that key, i.e the chords built on the notes of the scale of that key.
Use the melody notes to guide you as to which of those chord to select, (Most of the time the chord you want will contain the melody notes) bearing in mind the most important and most often used will be those built on the first fourth and fifth scale notes.
But don't be too restricted by these conditions - keep an open mind to other non standard ways that still sound good - and let your ear be the final judge.
what about if you don't have the notes of the melody figured out?
like you just made something up humming and want to find good chords that go along with it?
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If your ear isn't developed enough to 'feel' the appropriate chords, then the best thing is to figure out the notes of the melody on the guitar. - What's the last note of your melody? That's almost certainly going to be your key note and root of the last chord. Which chord sounds best with that note - major or minor? Then that's almost certainly your key chord (chord I).
Every key has a set of chords that naturally belong to it and those are the chords that will most likely be used in the song - but there are lots of exceptions, and in the end it's up to you.
If you don't know which chords belong to the key you've chosen - ask.
