The original Chang-hon Taekwondo pattern set was developed by General Choi Hong Hi, founder of the International Taekwondo Federation (ITF). The set contains 24 patterns, each named for a significant person or event in Korean history. The total of 24 patterns represents the number of hours in a day. Choi originally had developed only 20 patterns, but later he dropped Ko-Dang and added Eui-Am, Moon-moo, Ju-che, So-san, and Yon-gae. Ju-che is very difficult to perform so some organizations either do not use it or allow a substitution for older students.
This pattern set is one of the most difficult for new students since it uses intricate footwork and jumping kicks for color belt rank patterns (most schools using this pattern set use the first nine patterns for color belts). The black belt patterns are long (Yu-sin has 68 movements) with numerous jumping, spinning kicks, which make them physical and mentally demanding, especially during testing and competition.
The pattern set has been around for over 60 years and, due to the many different organizations and instructors that have used it, minor changes have been introduced into some of the patterns. Some of the changes may have been deliberate, but some we just mistakes that have been pasted down from instructor to instructor. The same may be said for where a kiyup is to be used; different organizations use them at different points.
For videos on all pattern sets, check out http://www.natkd.com/tkd_forms.htm