Minecraft wiki entry for Chunk_format, associated text: The "BlockLight" byte array is for block light levels. The "SkyLight" byte array stores the maximum sky light that reaches each block, regardless of current time. Each of them are 2048 bytes long, and both are omitted if no light reaches this section of the chunk. Light level is stored as 4 bits per block, 2 blocks sharing a byte : starting at 0, even blocks take the first nibble, and odd blocks the second one. This is the only known instance of small-endian data storage in Java Edition.