Well...

You could do a parallel harmony, which means you would play say a perfect 4th against every interval in your riff, regardless of what note it is. You can always play an interval you want against any note. It's the fact of whether you like how that harmony sounds against that interval that the given time.

If you wanted to play a minor 3rd to every interval in your melody, just locate the root, then come down a string and back (towards the nut) 2 frets and you got the minor 3rd.

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