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There are several national variants of ASCII. Therefore, the original ASCII is often referred as *US-ASCII*. The international standar _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. Thus, the characters that appear in those positions - including those in *US-ASCII* are somewhat "unsafe" in international data transfer.
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Thus, due to the "national variants" discussed above, some characters are less "safe" than others, for example, they might be transferred or interpreted incorrectly.
In Addition to the letters of the English alphabet ("A" to "Z" and "a" to "z"), the digits ("0" to "9") and the space , only the following characters can be regarded as really "safe" in data transmission:
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! " % & ' ( ) * + , - . / : ; < = > ?
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When naming files, variables, data files etc., it is often best to use only the characters listed above.
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The way to resolve this issue is to use the corresponding codes strictly for US-ASCII meanings; national characters are handled otherwise, giving them their own, unique and universal code positions in character codes larger than ASCII. But certain old softwares and devices may still reflect various "national variants of ASCII".