ISO 646 is an ISO standard that since 1972 has specified a 7-bit character code from which several national standards are derived. Since the portion of ISO 646 shared by all countries (the "invariant set") specified only those letters used in the basic modern Latin alphabet, other countries using the Latin alphabet with extensions needed to create national variants of ISO 646 to be able to use their native languages. 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.