The Rule of Two was the guiding principle of the Order of the Sith Lords. The philosophy, originated by Darth Bane when he founded the Order, stated that there could only be two Sith at one time. One Sith would embody the dark side's power, namely the Dark Lord of the Sith, while the second Sith was meant to crave it, that Sith being the Lord of the Sith.
Under the Rule of Two, the Sith master would train an apprentice knowing that the apprentice would one day kill the master and become the Dark Lord of the Sith. This tradition carried on until at least 41 BBY, when Darth Plagueis was killed by his apprentice. The new Dark Lord took on an apprentice of his own, Darth Maul.
- Star Wars: The Tragedy (First appearance)