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There are several national variants of ASCII. ThereforeAs a result, the original ASCII is often referred as *US-ASCII*. The international standard _ISO 646_ defines a character set similar to US-ASCII, but with code positions corresponding to US-ASCII characters {{@\[\]\{\|\}}} as _national use positions_. It also gives some liberties with 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 might be transferred or interpreted incorrectly.

\[[Kuhn 06|AA. C References#Kuhn 06]\] UTF-8 and Unicode FAQ for Unix/Linux
\[[ISO/IEC 646-1991|AA. C References#ISO/IEC 646-1991]\] ISO 7-bit coded character set for information interchange
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 5.2.1, "Character sets"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 3.2, "The character set and the corresponding encoding shall be documented," and Rule 4.1, "Only those escape sequences that are defined in the ISO C standard shall be used"
\[[Wheeler 03|AA. C References#Wheeler03]\] 5.4 File Names
\[[VU#881872|AA. C References#VU881872]\]