| _ ◌̲ | |
|---|---|
Underscore | |
| In Unicode | U+005F _LOW LINE U+0332 ◌̲COMBINING LOW LINE |
| Graphical variants | |
| ＿ | |
| U+FF3F ＿FULLWIDTH LOW LINE | |
| Different from | |
| Different from | U+0331 ◌̱COMBINING MACRON BELOW |
| Related | |
| See also | U+2017 ‗DOUBLE LOW LINE U+2381 ⎁CONTINUOUS UNDERLINE SYMBOL U+2382 ⎂DISCONTINUOUS UNDERLINE SYMBOL U+FE33 ︳PRESENTATION FORM FOR VERTICAL LOW LINE |
Anunderscore orunderline is a line drawn under a segment of text. Inproofreading, underscoring is a convention that says "set this text initalic type", traditionally used onmanuscript ortypescript as aninstruction to the printer. Its use to add emphasis in modern finished documents is generally avoided.[1]
The (freestanding)underscore character,_, also called alow line, orlow dash, originally appeared on thetypewriter so that underscores could be typed. To produce an underscored word, the word was typed, thetypewriter carriage was moved back to the beginning of the word, and the word wasovertyped with the underscore character.
In modern usage, underscoring is achieved with amarkup language, with the Unicodecombining low line or as a standard facility ofword processing software. The free-standing underscore character is used to indicate word boundaries in situations where spaces are not allowed, such as in computerfilenames,email addresses, and in InternetURLs, for exampleMr_John_Smith. It is also used asa proofreader's mark, to indicate that text should be italicised whentypeset, for instance_thus_ is to be rendered asthus.
The combining diacritic,◌̱ (macron below), is similar to the combining low line but is shorter. The difference between "macron below" and "low line" is that the latter results in an unbroken underline when it is run together: comparea̱ḇc̱ anda̲b̲c̲ (only the latter should look likeabc).[2][a]
In amanuscript (or typescript) to betypeset, various forms of underlining (seebelow) were therefore conventionally used to indicate that text should be set in special type such asitalics, part of a procedure known asmarkup. In printed documents underlining is generally avoided, withitalics orsmall caps often used instead, or (especially in headings) usingcapitalization,bold type or greaterbody height (font size).[1] Underlining may still be seen in display work.[3]
A series of underscores (like __________ ) may be used to reserve a blank space in text that is later to be filled in by hand, such as on a paper form. It is also sometimes used to create a horizontal line; other symbols with similarglyphs, such as hyphens and dashes, are also used for this purpose.
InGerman,Slovene and some otherSlavic languages, the underscore has recently gained prominence as the punctuation to formgender-neutral suffixes in gendered nouns and other parts of the speech.[4]
The underscore is also used in modern editions of Spanish vocal sheet music to indicateelision, instead of thebreve below (U+032E ◌̮COMBINING BREVE BELOW), which is less convenient to input on a computer.
In mathematical notations, underscores are sometimes used in the following contexts:
Inweb browsers, default settings typically distinguishhyperlinks by underlining them (and usually changing their color), but both users and websites can change the settings to make some or all hyperlinks appear differently (or even without distinction from normal text).[1]
As early output devices (Teleprinters,CRTs andline printers) could not produce more than one character at a location, it was not possible to underscore text, so early encodings such asITA2 and the first versions ofASCII had no underscore. IBM'sEBCDIC character-coding system, introduced in 1964, added the underscore, which IBM referred to as the "break character". IBM's report on NPL (the early name of what is now calledPL/I) leaves the character set undefined, but specifically mentions the break character, and givesRATE_OF_PAY as an example identifier.[7] By 1967 the underscore had spread toASCII,[8] replacing the similarly shaped left-arrow character,← (see also:PIP).C, developed at Bell Labs in the early 1970s, allowed the underscore in identifiers.[9]
Underscore predates the existence of lower-case letters in many systems, so often it had to be used to make multi-word identifiers, sincecamelCase (see below) was not available.
Underscores inserted between letters are very common to make a "multi-word" identifier in languages that cannot handlespaces in identifiers. This convention is known as "snake case" (the other popular method is calledcamelCase, where capital letters are used to show where the words start).
An underscore as the first character in anID is often used to indicate an internal implementation that is not considered part of theAPI and should not be called by code outside that implementation. InDart, all private properties of classes must start with an underscore; this usage is also common in other languages such asC++ even though those provide keywords to indicate that members are private. It is extensively used to hide variables and functions used for implementations inheader files. In fact, the use of a single underscore for this became so common that C compilers had to standardize on adouble leading underscore (for instance__DATE__) for actual built-in variables to avoid conflicts with the ones in header files.PHP "reserves all function names starting with __ as magical."[10]
Python uses names that both start and end with double underscores (so called "dunder methods", as indoubleunderscore) for magic members used for purposes such as operator overloading and reflection, and names starting but not ending with a double underscore to denote privatemember variables of classes which should bemangled in a manner which prevents them from colliding with members of derived classes unless the classes have the same name (__bar in classFoo will be mangled to_Foo__bar). By convention, members starting with a single underscore are considered private or protected, although this behavior only has inherent effect for modules, whereimport * statements by default import all names that do not start with an underscore, unless an export list is explicitly defined by the module.
A variable named with just an underscore often has special meaning.$_ or_ is the previous command or result in manyinteractive shells, such as those ofPython,Ruby, andPerl. InPerl,@_ is a specialarray variable that holds thearguments to afunction. InClojure, it indicates an argument whose value will be ignored.[11]
In some languages withpattern matching, such asProlog,Standard ML,Scala,OCaml,Haskell,Erlang, and theWolfram Language, the pattern_ matches any value, but does not performbinding.
The ASCII underscore character can be inserted with theentities_ or_ (or_ or_).
HTML has apresentational element<u> that was originally used to underline text; this usage wasdeprecated in HTML4 in favor of theCSS style{text-decoration: underline}.[12] In HTML5, the tag reappeared but its meaning was changed significantly: it now "represents a span of inline text which should be rendered in a way that indicates that it has a non-textual annotation".[12] This facility is intended for example to provide a red wavy line (or wiggly line) underline to flag spelling errors at input time but which are not to be embedded in any stored file (unlike an emphasis mark, which would be). Other styles are also available: doubled, dotted, and dashed.[13]
The elements may also exist in othermarkup languages, such asMediaWiki. TheText Encoding Initiative (TEI) provides an extensive selection of related elements for marking editorial activity (insertion, deletion, correction, addition, etc.).
Unicode has a free-standing underscore_ at U+005F, inherited from ASCII, which is a legacy of the typewriter practice of underlining using backspace and overtype. Modern practice uses thecombining diacriticU+0332 ◌̲COMBINING LOW LINE that results in an underline when run together: u̲n̲d̲e̲r̲l̲i̲n̲e̲. Unicode also hasU+0333 ◌̳COMBINING DOUBLE LOW LINE. In addition, there are single line and double line versions of thecombining macron below, a diacritic that applies to single letters only.[2]
| Effect | Using combining diacritic | Using html span style | Using macron below |
|---|---|---|---|
| single underline | a̲b̲c̲d̲e̲f̲g̲h̲i̲j̲k̲l̲m̲n̲o̲p̲q̲r̲s̲t̲u̲v̲w̲x̲y̲z̲0̲1̲2̲3̲4̲5̲6̲7̲8̲9̲ | abcdefghijklmnopqrstuvwxyz0123456789 | a̱ḇc̱ḏe̱ |
| double underline | a̲̲b̲̲c̲̲d̲̲e̲̲f̲̲g̲̲h̲̲i̲̲j̲̲k̲̲l̲̲m̲̲n̲̲o̲̲p̲̲q̲̲r̲̲s̲̲t̲̲u̲̲v̲̲w̲̲x̲̲y̲̲z̲̲0̲̲1̲̲2̲̲3̲̲4̲̲5̲̲6̲̲7̲̲8̲̲9̲̲ | abcdefghijklmnopqrstuvwxyz0123456789 | |
| single underline | A̲B̲C̲D̲E̲F̲G̲H̲I̲J̲K̲L̲M̲N̲O̲P̲Q̲R̲S̲T̲U̲V̲W̲X̲Y̲Z̲ | ABCDEFGHIJKLMOPQRTSUVWXYZ | A̱ḆC̱ḎE̱ |
| double underline | A̲̲B̲̲C̲̲D̲̲E̲̲F̲̲G̲̲H̲̲I̲̲J̲̲K̲̲L̲̲M̲̲N̲̲O̲̲P̲̲Q̲̲R̲̲S̲̲T̲̲U̲̲V̲̲W̲̲X̲̲Y̲̲Z̲̲ | ABCDEFGHIJKLMOPQRTSUVWXYZ |
Inplain-text applications, including plain-texte-mails where emphasis markup is not possible, the desired emphasis is often indicated by surrounding words with underscore characters. For example, "You must use _emulsion_ paint on the ceiling".
Some applications will automatically addemphasis to text manually bracketed by underscores, either by underlining or by italicizing it (e.g._string_ may render as eitherstring orstring).
Underline (typically red or wavy or both) is often used byspell checkers (andgrammar checkers) to denote misspelled or otherwise incorrect text.
Depending on local conventions, the following kinds of underlines may be used inline on manuscripts to indicate the specialtypefaces to be used:[14][15]
InChinese, the underline is a little-used punctuation mark for proper names (simplified Chinese:专名号;traditional Chinese:專名號;pinyin: zhuānmínghào; literally "proper name mark", used for personal and geographic names). Its meaning is somewhat akin to capitalization in English and should never be used for emphasis even if the influence of English computing makes the latter sometimes occur. A wavy underline (simplified Chinese:书名号;traditional Chinese:書名號;pinyin: shūmínghào; literally, "book title mark") serves a similar function, but marks names of literary works instead of proper names.[16]
In the case of two or more adjacent proper names, each individual proper name is separately underlined so there should be a slight gap between the underlining of each proper name.
Don't underline. Ever. It's ugly and it makes text harder to read. Underlining is another dreary typewriter habit... a workaround for shortcomings in typewriter technology.
Spacing Overscores and Underscores. U+203E OVERLINE is the above-the-line counterpart to U+005F low line. It is a spacing character, not to be confused with U+0305 COMBINING OVERLINE. As with all overscores and underscores, a sequence of these characters should connect in an unbroken line. The overscoring characters also must be distinguished from U+0304 COMBINING MACRON, which does not connect horizontally in this way.
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