Minecraft wiki entry for History, associated text: The file is separated into 4kb sectors (4,096 bytes). The first sector is the "chunk index" which defines where in the file and world a specific chunk is located. Every 4 bytes of the chunk index represents 1 chunk. Every 128 bytes of the chunk index represents a column of 32 chunks along the Z axis (North to South). Each subsequent 128 byte section represents 1 row of chunks along the X axis (West to East). But in a default world generated by the game, only 16 chunks are defined for each column and row.