- "Always two there are; no more, no less. A master and an apprentice."
The Rule of Two was a Sith philosophy mandating that only two Sith Lords could exist at any given time: a master to embody the power of the dark side of the Force, and an apprentice to crave it. The philosophy of two governed the Lords of the Sith for a millennium, beginning with its founder Darth Bane.
- "Created the Rule of Two, you did."
- ―Yoda, to the specter of Darth Bane
Even before Darth Bane created the rule, the Sith doctrines still rotated somewhat around the idea of two: a master and an apprentice. Some Sith temples could only be entered by a pair of Force-users. Eventually, Darth Bane would become the sole survivor of a war against the Jedi Order a millennium before the Clone Wars. Recognizing that it was Sith greed and in-fighting that led to their destruction, Bane created the Rule of Two so the Sith could operate in secret, using subterfuge to plot their revenge against the Jedi.
The Sith carried out their vendetta for a thousand years, gaining power in secret while the Jedi had believed that the Sith were destroyed. Darth Plagueis and his apprentice, Darth Sidious worked for years to coax the final secrets from the dark side. Sidious would call back on the wisdom of Plagueis as a planner and a prophet, knowing full well that two Sith were required to enact their means; one to serve as bait for the dark side, the other to be the vessel. Success would allow them the ability to harness the full power of the dark side and rule immortal for ten thousand years. After killing Plagueis in his sleep, the Sith finally revealed themselves during the Trade Federation's invasion of Naboo, after which they orchestrated the Clone Wars so Supreme Chancellor Sheev Palpatine—secretly the Dark Lord, Darth Sidious—could destroy the Jedi and transform the Galactic Republic into the Galactic Empire. Darth Sidious' apprentice, Count Dooku (Darth Tyranus) was slain by Anakin Skywalker who would become his new apprentice Darth Vader. Tyranus was slain to preserve the rule of two, allowing ancient Sith tradition to stand and also for Darth Sidious to have a more powerful asset than Tyranus whom he could use to form the Galactic Empire. The Sith ruled the galaxy for 23 years—far short of Sidious' dream of a ten thousand year Empire—until they were destroyed by a redeemed Anakin Skywalker—who sacrificed himself to kill Sidious during the Battle of Endor, thus ending the Rule of Two and the Sith and bringing balance to the Force.
- Darth Maul, Part IV (Indirect mention only)
- Star Wars: Episode I The Phantom Menace (First mentioned; albeit indirectly)
- Star Wars Journeys: The Phantom Menace (Indirect mention only)
- Star Wars: The Clone Wars – "Revival" (Mentioned only)
- Star Wars: The Clone Wars – "Eminence" (Mentioned only)
- Star Wars: The Clone Wars – "The Lawless" (Mentioned only)
- Star Wars: The Clone Wars – "Sacrifice" (First identified as Rule of Two)
- Ahsoka (Indirect mention only)
- Star Wars Rebels – "Always Two There Are" (Indirect mention only; in episode title)
- Star Wars Rebels – "Twilight of the Apprentice" (Mentioned only)
- Darth Vader 6: Vader, Part VI (Indirect mention only)
- Darth Vader 20: End of Games, Part I (Mentioned only)
- Star Wars: The Empire Strikes Back junior novelization (Mentioned only)
- Star Wars: Return of the Jedi junior novelization (Indirect mention only)
- "Authors of the Expanded Universe: Tales of the Sith"—Star Wars Insider 153
- Star Wars Rebels: Darth Vader, Rebel Hunter!
- Star Wars: I Am a Sith
- Star Wars: The Visual Encyclopedia