The first byte is used to detect transmission over a 7-bit channel--for example, email transfer programs often strip the 8th bit, thus changing the PNG signature. The 2nd, 3rd, and 4th bytes simply spell ``PNG'' (in ASCII, that is). Bytes 5 and 6 are end-of-line characters for Macintosh and Unix, respectively, and the combination of the two is the standard line ending for DOS, Windows, and OS/2. Byte 7 (CTRL-Z) is the end-of-file character for DOS text files, which allows one to TYPE the PNG file under DOS-like operating systems and see only the acronym ``PNG'' preceded by one strange character, rather than page after page of gobbledygook. Byte 8 is another Unix end-of-line character.

Text-mode transfer of a PNG file from a DOS-like system to Unix will strip off the carriage return (byte 5); the reverse transfer will replace byte 8 with a CR/LF pair. Transfer to or from a Macintosh will strip off the line feeds or replace the carriage return with a line feed, respectively. Either way, the signature is altered, and in all likelihood the remainder of the file is irreversibly damaged.

Note that the 9th, 10th, and 11th bytes are guaranteed to be 0 (that is, the ASCII NUL character) by the fact that the first chunk is required to be IHDR, whose first 4 bytes are its length--a value that is currently 13 and, according to the spec, will never change. (Instead, ``new chunk types will be added to carry new information.'') The fact that the 0 bytes in the length come first is another benefit of the big-endian integer format, which stores the high-order bytes first. Since NUL bytes are also often stripped out by text-mode transfer protocols, the detection of damaged PNG files is even more robust than the signature alone would suggest.