Originally Posted by: noticingthemistakeThe important thing to remember when harmonizing is that the harmonized part is complimentary to the original idea, but yet it can stand alone and sound good. Better results are from writting another part over the original without much regards to the interval used. Constant and variable interval harmonizations are good.

Any intervals except two are good to use. Those two are minor seconds/major sevenths, and tri-tones. They can be used but are usually to dissonant to produce any kind of correct musical phrasing.

However, if there are four guitars, you could have some wicked tritone harmonies.