Backslash
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First introduced in 1960 by Bob Bemer, the backslash, \, is a typographical mark (glyph) used chiefly in computing. Sometimes called a reverse solidus, it is the mirror image of the common slash or solidus. It is also known as a slosh, according to the Macquarie dictionary, rev. 3rd edn (meaning #4).
On Unix systems, and in Unix-affiliated programming languages such as C and Perl, the backslash is used to indicate that the character following it should be treated specially. It is sometimes referred to as the escape character, though this risks confusion with the character generated by the Esc key. In various regular expression languages it acts as a switch, changing literal characters into metacharacters and vice versa.
On DOS and Microsoft Windows computer systems, the backslash is used as a delimiter for directory names in file paths. This is in contrast to Unix and other directory systems such as internet addresses, which use the forward slash for this purpose. The backslash was chosen for path delimiter because in an early version – which did not support directories and thus had no need for a path delimiter – the slash was used to introduce command-line options (which are prefixed by a hyphen ("-") in Unix systems).
This association with computing might explain why the forward slashes in URLs are commonly (and erroneously) read as backslashes.
The backslash's prominence in Microsoft Windows pathnames has even led to it erroneously replacing the forward slash in contexts not relating to directory paths, or computers at all, for that matter. For example, people might write about an "African\American ancestry".
In the TeX typesetting system and in RTF files, the backslash begins a markup tag.
In many Unix applications (including but not limited to the GNU Emacs text editor and the Python programming language), the backslash appears at the end of lines that wrap around to the next line. This is related to its use as an escape character: it appears before the newline, thus removing the special significance attached to the character and allowing the text to carry on to the next line.
Image:CommandPromptJp.png
In the Japanese equivalent of ASCII, the code point that would be used for backslash is instead a yen mark (¥), while on Korean computer keyboards, the backslash corresponds to the won symbol (₩ or W). Many Japanese environments nonetheless treat it like a backslash, causing confusion. To add to the confusion, some fonts, like MS Mincho, render the backslash character as a ¥, so the unicode characters 00A5 (¥) and 005C (\) look almost identical, when these fonts are selected.
In mathematics a backslash-like symbol is used for the set difference.