“...power chords are played by placing one finger across two or three adjacent strings on the fretboard.”
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Okay I gotta crank my nitpicker up to 11 for a second...
What he’s describing isn’t a power chord. A textbook power chord uses two fingers on adjacent strings spaced a fret apart. This gives you the root note plus the fifth on top.
When you do the one finger trick across two strings like he describes, you’re playing the fifth plus the root on top. So the notes are sort of flipped. The sound is similar but isn’t technically a power chord IMO.
The riff in Smoke On The Water uses this kind of flipped, quasi power chord. The classic mistake is to play it with true power chords — an easy mistake to make because it’ll sound similar, but it won’t be quite right. Rick Beato actually has an episode about it:
https://www.youtube.com/watch?v=mX0IInspcgg
And if you put one finger across three strings then you’ve got the major third involved which is definitely not a power chord. It’s a major triad. An example of this would be the A chord in Rumble by Link Wray.
That's true if the guitar is tuned to standard tuning. For alternate tuning, especially for open-string tuning for slide guitar, since it's tuned to fifths, the Root would be on the "E" string and the Fifth would be on the same fret on the "A" string.
For example, a Power Chord with standard Tuning (E-A-D-G-B-E) at the fifth fret would be:
A D G C E A
A# D# G# C# F A#
B E A D G B
...but with Open or Slide tuning such as Open D (D-A-D-F#-A-D) the same A power chord would be played on the 7th fret of both the "E" and the "A" strings.
G D G B D G
G# D# G# C D# G#
A E A C# E A
Keith Richards in particular like to drop the E string to a D and his power chords were on the same fret across adjacent strings.