Dan Gun is the second pattern learnt for the ITF style of Tae Kwon Do. This pattern is required for the third grading, from yellow belt (8th kup) to green tags (7th kup), although the Tae Kwon Do grading system is cumulative so all previous patterns, (in this case, the first pattern, Chon Ji), are also required.
The definition of the pattern is as follows: "Dan Gun is names after the holy Dan Gun, the legendary founder of Korea in the year 2333 BC."
Dan Gun has 21 movements, and utilises a much greater number of techniques than Chon Ji. This pattern is distinctive because all nine of the punches are high section, as opposed to middle section which is far more common in other patterns.
The movements for the pattern, and its diagram, may be found at http://www.itf-information.com/sub0402.htm.