Also known as the Series of Related Worlds or the Twelve Related Worlds. Of all the worlds in the multiverse, there are twelve that are known and explored. These twelve worlds are further divided into more worlds, so each of the twelve main worlds is referred to as part of a numbered series. A new world comes about when there is some great event in history with more than one possible outcome. When that happens, the world splits, resulting one world with one outcome, and another world with another outcome. The differences between series are colossal events, such as the shifting of whole continents, while the differences between worlds within a series are more minute. What makes the Twelve Related Worlds related is that they all speak the same languages.
The oldest of the Related Worlds, home to the Conclave of Mages. In The Lives of Christopher Chant, it's stated that the Mages discovered the rest of the Related Worlds, and that's why the numbering starts with their Series.