chunks) is used in the associated MNG, JNG, and APNG formats. A chunk consists of four parts: length (4 bytes, big-endian), chunk type/name (4 bytes), chunk data (length bytes) and CRC (cyclic redundancy code/checksum; 4 bytes). The CRC is a network-byte-order CRC-32 computed over the chunk type and chunk data, but not the length. Chunk types are given a four-letter case sensitive ASCII type/name; compare FourCC. The case of the different letters in the name (bit 5 of the numeric value of the character) is a bit field that provides the decoder with some information on the nature of chunks it does not recognize. The case of the first letter indicates whether the chunk is critical or not. If the first letter is uppercase, the chunk is critical; if not, the chunk is ancillary. Critical chunks contain information that is necessary to read the file. If a decoder encounters a critical chunk it does not recognize, it must abort reading the file or supply the user with an appropriate warning. The case of the second letter indicates whether the chunk is "public" (either in the specification or the registry of special-purpose public chunks) or "private" (not standardised). Uppercase is public and lowercase is private. This ensures that public and private chunk names can never conflict with each other (although two private chunk names could conflict). The third letter must be uppercase to conform to the PNG specification. It is reserved for future expansion. Decoders should treat a chunk with a lower case third letter the same as any other unrecognised chunk. The case of the fourth letter indicates whether the chunk is safe to copy by editors that do not recognize it. If lowercase, the chunk may be safely copied regardless of the extent of modifications to the file. If uppercase, it may only be copied if the modifications have not touched any critical chunks. Critical chunks A decoder must be able to interpret critical chunks to read and render a PNG file. IHDR must be the first chunk; it contains (in this order) the image's width (4 bytes) height (4 bytes) bit depth (1 byte, values 1, 2, 4, 8, or 16) color type (1 byte, values 0, 2, 3, 4, or 6) compression method (1 byte, value 0) filter method (1 byte, value 0) interlace method (1 byte, values 0 "no interlace" or 1 "Adam7 interlace") (13 data