The dan (段) ranking system is used by many Japanese, Okinawan, and Korean martial art organizations to indicate the level of a person's ability within a given system that could be generally described as a finite set that once achieved becomes an art that can be defined by some formalized system of patternism. Used as a ranking system to quantify skill level within a given set of specific patterns, it was originally used at a go school during the Edo period. It is now also used in most modern Japanese fine and martial art.