The Known Worlds were a vast collection of parallel universes (PROSE: Warlords of Utopia, The Cosmology of the Spiral Politic) or alternate timelines discovered and explored by Marcus Americanius Scriptor. He broadly divided them into "Romas" and "Germanias", the former referring to Earths on which the Roman Empire never fell, the latter referring to Earths on which Nazi Germany won World War II. The two groups were separated by the Divide; Scriptor suspected that more clusters existed beyond further Divides. (PROSE: Warlords of Utopia)
According to one account, some or all of the Known Worlds were created by the Great Houses as part of the Second Time Front during the War in Heaven. (PROSE: The Brakespeare Voyage)