Definition: "Always two there are, a master and an apprentice," Yoda said of the Sith in The Phantom Menace. This system of Sith teaching, known as the Rule of Two, is our only view of the Sith during the Star Wars films. Viewers may not realize, therefore, that the Rule of Two represents a major overhaul of Sith teachings and practices, and how much those teachings and practices change over the course of the Expanded Universe.
In-universe: The Sith originated as a Force-sensitive alien species from the planet Korriban, whose culture developed philosophies and rituals of the dark side. When exiled Dark Jedi encountered the Sith after 6,900 BBY, they overthrew the king and declared themselves rulers, eventually forming a great Sith Empire.
The idea of many Sith existing at a time was standard for thousands of years. Over time, this caused great conflict; particularly during the New Sith Wars, the Sith were plagued by infighting that prevented them from presenting a strong, united front against the Jedi and the Republic.
Lord Kaan's Brotherhood of Darkness, established around 1,010 BBY, sought to unite the Sith and prevent further infighting. Kaan took the title "Dark Lord" and prohibited anyone from using the title "Darth," believing it would lead to further conflict and infighting.
Bane, a member of the Brotherhood of Darkness, believed that there was a better way to strengthen the Sith. In 1,000 BBY, he unleashed a thought bomb, a focus of the destructive power of the dark side, on Ruusan. This act destroyed the Brotherhood of Darkness and many Jedi who fought them.
As the last of the Sith, Bane reestablished the title Darth and created the Rule of Two. Under this tradition, only two Sith could exist at a time: a master and an apprentice. The culmination of the apprentice's training was to kill her master and take his place, training her own apprentice. In this way, the Sith line would only continue to grow more powerful.
The Rule of Two lasted for over a thousand years. Although there were some dark side users who called themselves Sith outside of the Rule of Two tradition, the Sith master and apprentice were adamant that these were impostors and wanna-bes, only Dark Jedi and not true Sith. For example, Count Dooku (Darth Tyranus) trained Asajj Ventress during the Clone Wars, but told her: "You are not Sith. You wear the trappings of the Sith, you fight like the Sith, but this can be imitated, however. You lack a vital quality found in all Sith. Sith have no fear, and I sense much fear in you."
The Rule of Two ended with Darth Krayt's Sith Empire in 130 ABY. Krayt established the Rule of One, which focused on obedience and loyalty to the head of the Sith Order rather than rising up to become the most powerful Sith.
- Jedi vs. Sith: The Essential Guide to the Force by Ryder Windham (2007)
- Darth Bane: Path of Destruction by Drew Karpyshyn (2006)