Watch the Harmony Lines online guitar lesson by Jeff Scheetz from Street Theory for Guitarists
So, defining "harmony": In general, it can be thought of as what is produced by the combination of notes that create a chord. So, when you have multiple notes happening at the same time, that's harmony. This lesson is focused on creating a harmony line over a melody. As a guitarist, I always love hearing a cool harmony line, whether it's the Allman Brothers, or Iron Maiden, it's always a great sound to be able to add a harmony line in a melody or a solo.
First, let's look at the basic premise of creating a harmony line. You're simply taking a single note melody line, and adding additional notes to that line, which will in effect create chords. So, knowing your "chord theory" is necessary here. For example: While you could play a note and then play any other note to go with that, it will sound best if you play an interval that would be in that notes chord.
We're going to play a "diatonic" harmony, which means all the notes are from the same scale. So, if I'm playing a line in my G major scale, and I start with a single G note, a great note to play as a harmony to that would be the B note. Why? Because that note is a 3rd from G, and we know that the 3rd is the first note we add as we're creating a chord.
You can create a harmony line above OR below the melody line. You can have both!