A sith law making it so that there can never be more than two active sith at one time, one master, and one pupil. The rule was set in place by Darth Bane after he killed all the other sith.(Darth bane also started the tradition of prefixing sith names with "darth")
Darth Sideous only ever had one deciple at a time, first Darth Maul, then Darth Tyranus, then Darth Vader. Although he surounded himself with other Dark Jedi, there were only ever two Sith at one time, in accordance with the Rule Of Two.
by SlackerApathy August 19, 2005