The Clan System was a simple team tournament system on KGS where in each roughly 3 month period, teams ("clans") played games to reach a certain minimal level of activity and compete in the Clan Brawl (a big team tournament). The first of the clans PH1 was founded in August 2005, the system was active until 2011.
There were normally a number of other tournaments throughout the period including a rengo and individual tournament.
There is a room on KGS, "♦ The Battlefield", where all clan related activities took place. The clan rules, statistics and forum were last available at http://www.kgsclans.co.uk/ (the page is defunct as of April 2013).