Power chords can be found in almost every musical style like rock, hard rock, metal, punk, blues rock, blues, country. I'm sure we could even find them in classical or jazz if we looked hard enough. The style of music I think of right away when I think of power chords is 70's - 2000's era popular rock styles. Power chords paired up with distortion can have a very full and heavy sound giving us that dirty and gritty bottom end guitar sound. Except for blues and country, we mostly hear power chords played with varying levels of distortion from light to very saturated.
A power chord is a 2-note chord consisting of the root and the 5th note of its major scale. Classical music theory does not recognize 2 note harmonies as chords and only recognizes 3 notes or higher to be proper chords. To me, and many other people a power chord is a real chord and for the purposes of this lesson we will assume that it is.
The proper name for an E power chord would be "E5". The chord name tells us the root "E" and the note that accompanies it "5" meaning the 5th note of the E major scale. For example, here is the E major scale: (E, F#, G#, A, B, C#, D#). E and B would be the 2 notes of an E5 power chord.
If the power chord was an "A5", then that would tell us that the root is A and that we need to add in the 5th note of the A major scale to complete the chord. For example, here is the A major scale: (A, B, C#, D, E, F#, G#). A and E would be the 2 notes of an A5 power chord.
That's it! That is the basic theory behind a power chord.
Open Power Chords - There are a few ways to play power chords that can make them sound different. In Fig #1 we look at open power chords. These are power chords that use an open string for the root note like E, A or D.
Fretted Power Chords - In Fig #2 we can see that both notes are fretted with no open strings. These power chords can easily be moved around the next and even slide from one to another.
3 String Power Chords - In fig #3 we look at three string power chords. These power chords still only use 2 notes except they add in a higher octave root note. For a B5 which is contains the notes B and F# the three-string version would break down like this: B, F#, B. There are two B notes which are a complete octave apart. This adds a fuller sound to the power chord but can also take away a tiny bit of its hard hitting grit.
Inverted Power Chords - Then there are inverted power chords! Inverted power chords still only use 2 notes, but the lowest note is no longer the root note. These chords can sound a lot darker and can be found in songs like "Crazy Train", "Smells Like Teen Spirit", Say It Ain't So", and many Foo fighters and Green Day songs.
In fig #4 we have another B5 power chord except it is inverted. A B5 normally is played with a B in the bass position (lowest note) and an F# as the secondary note in a higher position. For an inversion the B would no longer be in the lowest position and instead the F# would be. The chord now reads as: F#, B, F#, B as per fig #4.
Other types - there are other types of power chords like "Drop D" Power chords but we don't cover them in this lesson.