The Parallel World theory [presents] the idea that ghosts may sometimes live in a universe reflecting a former time, which sometimes overlaps with ours. [Above, top/right is] a typical explanation of this idea from a 1978 book for kids, World of the Unknown: All About Ghosts […] The motif forms the basis of certain ghost stories, such as M. R. James’ Number 13.  In this story, an old Danish hotel, occupying what once was a medieval building, has a room 12 and a room 14 next to each other, the 13 omitted for superstitious reasons […] Each is a large room with three windows at one end.  Ah, but when night falls, a room 13 mysteriously appears between the two. 12 and 14 grow narrower and lose each a window to number 13. There are weird sounds and grotesque silhouettes to be seen opposite that particular room on the wall of an adjacent building where the light shines out through the hotel’s windows onto the blank surface…” from The Ghostland Around Us, Beneath Us