Minecraft wiki entry for Nether_portal, associated text: A nether portal is built as a vertical, rectangular frame of obsidian (4×5 minimum, 23×23 maximum). The four corners of the frame are not required, but portals created by the game always include them, resulting in 4 free/extra obsidian. The obsidian can be placed in any manner, e.g. by placing mined obsidian, by completing a ruined portal, or by casting it in place using lava and water. Adjacent portals can share obsidian blocks. A nether portal cannot be built horizontally like an end portal.