Rule Two of the Sith

The Rule of Two was instituted by Darth Bane, once a student at Kaan's Sith academy, in response to what he viewed as inevitable, self-destructive infighting within the Sith Order. The Rule of Two states that there would be only two Sith at one time, a Master and an Apprentice, guaranteeing that when the Apprentice becomes powerful enough in the ways of the Dark Side to take the title of Master, only then would they be worthy of the title. This transfer of power would only take place when the Apprentice takes their Master's life and finds a worthy student to repeat the cycle. Both Master and Apprentice were considered to be Sith Lords.

Some Notable Lord/Apprentice

-Darth Sidious and Darth Vader

-Darth Sidious and Darth Maul

-Darth Tyranus and Ventress

-Darth Plagiues and Darth Sidious

-Vindican and Darth Malgus

-Etc.