Minecraft wiki entry for Nether_portal, associated text: For players, if no portals exist in the search region, the game creates one, by looking for the closest suitable location to place a portal, within 16 blocks horizontally (but any distance vertically) of the player's destination coordinates. A valid location is 3×4 buildable blocks with air 4 high above all 12 blocks. When enough space is available, the orientation of the portal is random. The closest valid position in the 3D distance is always picked.