Minecraft wiki entry for Nether_Fortress, associated text: Nether fortresses generate in all Nether biomes. To do so, the game splits the Nether into regions in which one of either a fortress or a bastion remnant can generate. The regions are 432×432 blocks in Java Edition and 480×480 blocks in Bedrock Edition. Each region has a 4 chunk separation located on the south and east borders of the region in which neither a fortress nor a bastion can generate. This leaves only a 368×368 block section in Java Edition or 416×416 block section in Bedrock Edition where a structure can generate. Two structures never generate in the same region, although they might overlap if they generate close to the separation border. In Java Edition, the chance of a fortress generating instead of a bastion is 2⁄5 (40%), while in Bedrock Edition the chance of a fortress generating instead of a bastion is 1⁄3 (33.3%).