ISO/IEC 646

ISO/IEC 646

ISO/IEC 646:1991, Information technology — ISO 7-bit coded character set for information interchange, is an ISO standard that since its first edition in 1972 has specified a 7-bit character code from which several national standards are derived. ISO/IEC 646 was also ratified by ECMA as ECMA-6.

Since the portion of ISO/IEC 646 shared by all countries (the "invariant set") specified only those letters used in the ISO basic Latin alphabet, other countries using additional letters needed to create national variants of ISO 646 to be able to use their native scripts. Since universal acceptance of the 8 bit byte did not exist at that time, the national characters had to be made to fit within the constraints of 7 bits, meaning that some characters that appear in ASCII do not appear in other national variants of ISO 646.