The underscore [ _ ] (also called understrike, underbar, low line, or low dash) is a character that originally appeared on the typewriter and was primarily used to underline words. To produce an underlined word, the word was typed, the typewriter carriage was moved back to the beginning of the word, and the word was overtyped with the underscore character.
This character is sometimes used to create visual spacing within a sequence of characters, where a whitespace character is not permitted, e.g., in computer filenames, email addresses, and in Internet URLs. Some computer applications will automatically underline text surrounded by underscores: _underlined_ will render underlined. It is often used in plain text-only media (IRC, instant messaging, classical Email) for this purpose. When the underscore is used for emphasis in this fashion, it is usually interpreted as indicating that the enclosed text is underlined (as opposed to being italicized or bold, which is indicated by /slashes/ or *asterisks*, respectively).
The underscore is not the same character as the dash character, although one convention for text news wires is to use an underscore when an em-dash or en-dash is desired, or when other non-standard characters such as bullets would be appropriate. A series of underscores [like ___-______ ] may be used to create a blank to be filled in on a form, although the resulting blank may have tiny gaps between each individual underscore. It is also sometimes used to create a horizontal line; other symbols with similar graphemes, such as hyphens and dashes, are also used for this purpose.
Unicode encoding and typing
In Unicode, the character is encoded at U+005F _ LOW LINE (HTML
_ · spacing character). The ASCII value of this character is 95. On the standard US or UK 101/102 computer keyboard it shares a key with the hyphen-minus (-) on the top row, to the right of the 0 key.
A similar Unicode character is U+2017 ‗ DOUBLE LOW LINE, which is an underscore with two horizontal lines instead of one.
The underscore is used as a diacritic mark, "combining low line", in some Egyptian languages, some languages using the Rapidolangue orthography in Gabon, Izere in Nigeria, and Native American languages such as Shoshoni and Kiowa
Usage in computing
The common punched-card character sets of the early 1960s had no lower-case letters and no special character that would be adequate as a word separator in identifiers. IBM's EBCDIC character-coding system, introduced in 1964 at the same time as the IBM System/360 computer series, added the underscore, which IBM referred to as the break character, but not lower case (later editions of EBCDIC added lower case). IBM's report on NPL (the early name of what is now called PL/I) leaves the character set undefined, but specifically mentions the break character, and gives RATE_OF_PAY as an example identifier. By 1967 the underscore had spread to ASCII, replacing the similarly-shaped left-arrow character (←) previously residing at code point 95 (5F hex) in ASCII-1963 (see also: PIP). C, developed at Bell Labs in the early 1970s, allowed the underscore as an alphabetic character.
Underscores inserted between letters is very common to make a "multi word" identifier in languages that cannot handle spaces in identifiers. The only other popular alternative is CamelCase where each word starts with a capital letter but nothing separates them.
A common programming convention in various programming languages is to use a leading underscore in a name (for example _name) to indicate that the name is intended for internal use within a library (computing) or a header file and does not appear in an API. This convention has also been used for internal variables that are produced directly by the compiler or interpreter (rather than being set by the header file), sometimes called pseudo-environment variables. Sometimes more than one underscore is used to indicate internal variables ("PHP reserves all function names starting with __ as magical."), or
__DATE__ in C compilers.
A variable named with just an underscore often has special meaning. In many shells
$_ is the previous command. In the interactive Python interpreter, the variable
_ holds the result of the previous evaluation, this is also true of Ruby, Perl and Windows PowerShell where
$_ is the previous result. In Perl
@_ is a special array variable that holds the arguments to a function.
|Look up underscore in Wiktionary, the free dictionary.|
- NPL Technical Report (PDF). IBM. 1964. p. 23. Retrieved 9 June 2011.
- Fischer, Eric. "The Evolution of Character Codes, 1874-1968" (PDF). Retrieved 9 June 2011.
- Ritchie, Dennis (c. 1975). "C Reference Manual" (PDF). Retrieved 9 June 2011.
- Conn, Rex C.; Rawson, Tom; Paul, Matthias; Dye, Charles; Georgiev, Luchezar (2002-02-27). 4DOS 8.00 online help.