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There are several national variants of ASCII. As a result, the original ASCII is often referred as US-ASCII. ISO/IEC 646-1991 defines a character set, similar to US-ASCII, but with code positions corresponding to US-ASCII characters {{@\[\]\{\|\}}} as _national use positions_ \[[ISO/IEC 646-1991|AA. Bibliography#ISO/IEC 646-1991]\]. It also gives some liberties with the characters {{\#$^`\~}}. In ISO 646, several national variants of ASCII have been defined, assigning different letters and symbols to the national use positions. Consequently, the characters that appear in those positions, including those in US-ASCII, are less portable in international data transfer. Consequently, due to the national variants, some characters are less portable than others --- they others—they might be transferred or interpreted incorrectly.

As a result of the influence of MS-DOS, file names of the form {{xxxxxxxx.xxx}}, where x denotes an alphanumeric character, are generally supported by modern systems.  On some platforms, file names are case sensitive; while on other platforms, they are case insensitive.  VU#439395 is an example of a vulnerability resulting from a failure to deal appropriately with case sensitivity issues \[[VU#439395|AA. Bibliography#VU439395]\].