Power chords are essentially double stops, or partial chords. True chords have at least three different notes. Power chords contain only the root and 5th note of a scale. But because they are used like chords they are considered chords. They are usually notated as a 5 chord. For example, a C Power chord is usually notated C5. There are both two and three string versions of the Power chord. The three string version simply adds the octave of the root note.