The “power chord” is a simplified chord, used mostly by rock guitarists but it also has a place on the piano.
Remember that a major chord consists of the first, third and fifth degrees of the major scale. A minor chord is like a major chord but with the 3rd lowered a half-step.
A power chord, however, just has the 1 and 5 and omits the 3rd. Because we leave out the 3rd in a power chord, it is neither major nor minor.
You can play a power chord whenever a major or minor chord is required. In fact, because the 1 and 5 are present in every chord except for diminished and augmented chords, you can substitute power chords almost everywhere.
The reason rock guitar players love power chords is that you only have to learn a single handshape in order to play all possible power chords. Also, when you apply a lot of distortion to the sound, power chords sound better than full chords.
Power chords are not very common in piano music. But they are useful if you want to play chords way down low on the keyboard.
With those low tones, adding the 3rd makes the sound too muddy, so playing just 1-5 will sound better than 1-3-5.
The notation for a power chord, for example the C power chord, is C5. Less common is something like C(omit3).
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