Minecraft wiki entry for Nether_portals, associated text: The Nether portal spawning algorithm can only spawn a portal within a 33×33 block column centered on the destination, but it does scan that width (and the height of the world) for open space to place the portal. This will often cause it to spawn a portal at a location significantly different than the corresponding location in the other world. The larger the distance between a portal and its "ideal" destination, the larger the zone of exclusion. This zone is the area in each world where you cannot build another portal without breaking the link between the first two portals. One way to think of this zone is as spheres around each portal, each of a true radius equal to its distance to the equivalent location of the other. For example, if the Overworld portal was at (0,50,0) and the Nether portal at (0,100,0), then the portals are 50 meters away from each other. In this (simple) case, if a Nether portal was built closer than 50 meters to (0,50,0), then the Overworld portal will now link to it.