About
The P5 interval is, next to the perfect unison, one of the most constant intervals in music. It has a solid foundation on which to build and is therefor used in nearly every chord imaginable. Except of course when it's a "no5" chord. Like watching video's? Check out our video about the perfect fifth!
But next to giving a solid foundation to a chord there aren't a lot of other reasons to include it. And thus it is also excluded (or not always included) from a chord in many musical styles.
Listen to some examples of the P5 interval!