The main location in the Chrestomanci books. It can be assumed that the universe in Witch Week is a bridge between 12 A and 12 B because it is mentioned as being like a stripe in a rainbow that is still joined to the main rainbow at both ends, and Chrestomanci mentions that Janet's world and that world are so much alike.
This is the main world in the books. It is Chrestomanci's world, and where Cat and the others all live. It is similar to our world (Twelve B), but much more old-fashioned. They use magic rather than rely on science, and a large portion of the population can use magic. For more information on the types of magic in Twelve A, go here.
Notable differences between our worlds is that America does not exist in Twelve A. Rather, Atlantis was never colonised and is now run by the Native Americans.