Power chords are also called root-fifth or fifth chords because they use the root note and 5th note of a scale. Another way to refer to the D power chord is D5.
 |
D5 Chord, power chord on 6th string |
 |
D5 Chord, power chord on 5th string |
 |
D5 Chord, power chord on 4th string |
 |
D5 Chord, power chord on 3rd string |