Power Chords
Based on E Chord Form
Power chords based on the A form.
Power chords are typically a rock guitar type thing. However, the term can be used for piano and other chordophonic instruments, and by other styles than rock (such as jazz or pop or folk, etc.)
What are they exactly? This an interesting question. We'll define them with 3 ideas in mind (what we'll call the 3 criteria): voicing, relative strength of the sound, and their application (style usage) - see below. These are subjective lines we draw for ourselves.
The 3 Criteria
Voicing: Typically these chords have only a root and 5th in the bass of the chord - creating an interval of a 5th (a Perfect 5th - P5). In this instance (and in the example above), the root is the lowest tone, and the 5th (the tone 5 up) is stacked on top of this root, on the next string. These chords are known as R5 chords (Root-5th). They are a fragment of a larger form. In this case, the E form.
It sometimes happens that players use the tones in the opposite stacking order too (5th as lowest note, then the root). . In this instance, the interval is 4th (Perfect 4th - P4), yet this 'isn't as powerful' (see Relative Strength below).
We can also add another root to the power chord, to create an octave, with the 5th in the middle.
The simplest definition of a power chord may just be: any chord with a root/5th in the bass of the chord. For this definition, full versions or non-fragmented chords, such as your basic E, could be called a power chord (since what we call a power chord - R5 - is within the full version). The full version of an E or A form moving up the fretboard could also be called the same (they have the R5 in the lowest tones).
Relative Strength of Sound: A power chord should stand up to its name. It should be powerful. Yet, where does the line get drawn between power and not power? Typically, for the strength to get the name, it must be in the lower range of the frequency spectrum.
So your question here is, at what pitch level is a chord with a P5 (or P4) no longer a 'power chord'?
Also, to get the strength, does your distortion have to be cranking? Not necessarily. Acoustic guitars (without distortion) utilize these types of chords.
Application (Style Usage): These chords are typically used in rock and punk music. Most Green Day, Scorpion, and Ramones songs use them. Jazz players use R5 chords sparingly.
If a jazz song happens to use a 'power chord', they wouldn't necessarily be called that (they might just be called a voicing). Similar to hammer-ons (hammers) and pull-offs (pulls) in classical guitar, simply being called slurs.
In summary, we use the term power chord, most commonly, to refer to chords with a root and 5th only, where there is a strong strength of sound, and used in rock and punk guitar music. Every guitar style has particular voicings which define its overall sound.
