The Stationmasters' Guide Book: An History and Compendium

In the multiverse, there are always two definitive versions of one particular universe. (Of course, there are all the parallel verses and their associated branchings-off, but they are shadow worlds, unreal and without body, unless there is a serious train-wreck in the workings of… things… and one of the shadow worlds, in an extreme emergency, is shunted onto the track the previous verse was derailed from. All this is managed by The Stationmasters.) In these two definitive versions of a smaller verse within the larger multiverse, one of these worlds will have magic, and one will not. Traffic between them is generally discouraged by The Stationmasters. Sometimes, though, it is possible, by dint of dogged perseverance and cat-like patience, to obtain an inter-dimensional passport. One of the most extraordinary instances of such a document being issued involves the sister verses of Deorsa and Earth.
The Stationmasters' Guide Book: An History and Compendium, page 1 (sometimes), paragraph 1 (nearly always)