If you play guitar, you will probably know this chord as a powerchord. It only has two notes, the perfect prime (perfect unison) and the perfect fifth. The chord can never be major or minor, because it does not contain a third.
Usage and play
The powerchord is a very strong sounding chord and is being used a lot in rock, punk and metal to lay out a solid and heavy foundation. The main reason why this chord sounds so heavy is because there is little to no tension between the two notes being used. For instance, if you try to play a C and a Db (or C#) at the same time (which is not really a chord) you can hear tension between the notes, they have a great necessity to resolve and release tension. Play a C and a G together sounds very monochrome, almost as if it is the same note. This makes the chord very solid.
The powerchord is relatively easy to play on both piano and guitar. But then again, anything at higher speeds becomes more complex.
Shifting this chord using a pentatonic minor scale (or blues scale) as tonics actually creates a lot of dissonant notes. However, because the chord sounds so strong and the shifted notes pass along quite rapidly it sounds very natural.
Examples
Nirvana - Smells Like Teen Spirit
Eric Clapton - Cocaine
Deep Purple - Smoke on the Water
The White Stripes - Seven Nation Army
