Summary
The backslash is a typographical mark used mainly in computing and mathematics. It is the of the common slash . It is a relatively recent mark, first documented in the 1930s. It is sometimes called a hack, whack, escape (from C/UNIX), reverse slash, slosh, downwhack, backslant, backwhack, bash, reverse slant, reverse solidus, and reversed virgule. efforts to identify either the origin of this character or its purpose before the 1960s have not been successful. The earliest known reference found to date is a 1937 maintenance manual from the Teletype Corporation with a photograph showing the keyboard of its Kleinschmidt keyboard perforator WPE-3 using the Wheatstone system. The symbol was called the "diagonal key", and given a (non-standard) Morse code of . (This is the code for the slash symbol, entered backwards.) In June 1960, IBM published an "Extended character set standard" that includes the symbol at 0x19. In September 1961, Bob Bemer (IBM) proposed to the X3.2 standards committee that , and be made part of the proposed standard, describing the backslash as a "reverse division operator" and cited its prior use by Teletype in telecommunications. In particular, he said, the was needed so that the ALGOL boolean operators (logical conjunction) and (logical disjunction) could be composed using and respectively. The Committee adopted these changes into the draft American Standard (subsequently called ASCII) at its November 1961 meeting. These operators were used for min and max in early versions of the C programming language supplied with Unix V6 and V7. The Teletype Model 33 (1963) appears to be the first commercially available unit that has the character as a standard keytop for sale in some markets; this model has a full ASCII character set. In many programming languages such as C, Perl, PHP, Python, Unix scripting languages, and many file formats such as JSON, the backslash is used as an escape character, to indicate that the character following it should be treated specially (if it would otherwise be treated literally), or literally (if it would otherwise be treated specially).
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