Originally Posted by: Geoff WalkerHey there - now I'm really confused -
I thought that "power chords" were the first and the fifth NOTE of a scale - not separated by 5 semitones, so for example 5th string 3rd fret (C), and 4th string 5th fret (G), and if you were going to go REALLY mad and make it a three note chord you could put in the octave of the first note - 3rd string 5th fret (C again).
If you are only using two notes is it still a DIAD - meaning two notes, rather than TRIAD meaning three notes.
Help.

Maybe this will clear things up. It was somewhat confusing by how it is worded. (Power Chord = 3 notes, A-E-A) In reality there are only 2 notes being used; A and E (This is a Diad). Always remember that any scale is a division of the Octive into smaller Intervals by various degrees. The maximum division of Intervals that will fit from any given note to it's Octive is 12.
Any two pitches--played simultaneous or consecutively ---constitute an Interval that can be identified by either of two methods; Tonal Context -----
Which is what you used above referring to the first and fifth intervals in Scale degrees from the Root (it's my preferred choice to use as well) and the other is by Pitch Class set which is used to measure the interval in semitones by count. I believe the Pitch Class method will be much easier to explain the theory behind what's going on here.
Many players memorize Whole-step, half-step (Or Whatever) patterns to form scales, but never grasp the function in Tonal context that the intervals create for the relationship with the notes.
Let's create a Diatonic scale (a 7 note scale). We'll use the "A" Major scale since "A" is Tonic for the Diad above .

6thxx|--|--|--|--|-A-|--|-B-|--|-C#|-D-|--|-E-|--|-F#|--|-G#|-a-|

If this were on the 6th string, ( 6thxx| = Open E) would be the Nut and A would rest between the 4th and 5th frets.
It's important to realize that scales form in both directions from the Tonic (A) but the interval sequence switches it's players around (So to speak) so that their relationship remains the same with the Tonic.
We know that E is the 5th interval (Dominant) of A. If we count in sequence From A to E by semitones, it equals 7 intervals. Now if we count again from A going in the opposite direction using 7 semitones, it doesn't work. We end up on D, which is the 4th interval (Sub-Dominant) of A.. Why??? Because 7 forward and 7 backward = 14 intervals and the Max is 12. If we count back 5 semitones from A, we end up on the open E and that works because 7+5=12.
Now for that problem created counting back 7 intervals and ending up on D.
If we count up from A to D by semitones, it equals 5 intervals and we're equal again to 12 (5 up and 7 back). So the intervals created to make the Perfect 4th up from the Tonic become the intervals needed to create the Perfect 5th down from the Tonic and vise-versa.
The Original Post displays a tab with a Diad using the notes 6th string, 3rd fret (G) and 5th string, 3rd fret (C) being played together, asking "What is this"??? Well, C is 5 semitones up from G which makes C the Perfect 4th interval of G. What if we count back 5 semitones from a note like we did from A? We found the 5th (E). So if we count back 5 from C, we find it's 5th (G). If we count up 7 from C, yep it's G, the 5th of C which makes C the Tonic, so the tab creates a C Diad in tonality and even though the G is lower in tone, it belongs to C.
Count back 5 semitones from D in the A Major scale above. Yep, A is the 5th of D. Now count up 7 semitones from D, A again and it equals 12.....
If you play the open low E and the open A string together, A wins, you can even throw in the e Octive if you'd like (4th string, 2nd fret) still an A. You guessed it, A is the Perfect 4th of E. It's all in the intervals.
I probably confused things beyond belief.
Hope it helps...........................