Originally Posted by: ryrysweetiepiehow do you do a harmony with 2 guitars? like, the way children of bodom does on all their songs with the guitar and the keyboard?

All the other responses were good but didn't say anything about how you would know whether to use a minor or major third, or a diminished or perfect fifth for example. Of course you can use your ear, but there is a purely theoretical way too.

Say you are playing in Em, and your melody is built on notes from the natural minor scale in E only. Say you are harmonizing in thirds. To find the notes you want to harmonize with, simply start from the note you already have written in the melody, and go up two steps within the scale, a third up, and voila, there's your harmonizing note. Do this with each of the notes you wish to harmonize and remember to stay within the E natural minor scale and you will automatically get minor or major thirds where they sound good!