Worldwalls

The boundaries between physical universes. The population density of a given area affects the permeability of this boundary; in areas where very many living beings are crowded into a small space, the pressure of their needs and desires "frays" the structure of spacetime somewhat. If the number of beings exceeds a certain threshold value, spontaneous worldgates may open in the area of greatest concentration.

The danger of this condition is offset by the ease with which wizards aware of the worldgates can control and manage them, their spells being made more effective by the same population pressure. The gates can sometimes even be made permanent, like the gates at New York's Penn Station and Grand Central Terminal, and at Chur in Switzerland and London-Tower Hill. (SYWTBAW et seq.)

(See also: Worldgating (general theory).)