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Term for the collection of horizontal and vertical spaces ('blank characters') in a character set
In computer programming, whitespace is any character or series of characters that represent horizontal or vertical space in typography. When rendered, a whitespace character does not correspond to a visible mark, but typically does occupy an area on a page. For example, the common whitespace symbol SPACE (also ASCII 32) represents a blank space punctuation character in text, used as a word divider in Western scripts.
With many keyboard layouts, a horizontal whitespace character may be entered through the use of a . Horizontal whitespace may also be entered on many keyboards through the use of the key, although the length of the space may vary. Vertical whitespace is a bit more varied as to how it is encoded, but the most obvious in typing is the result which creates a 'newline' code sequence in applications programs. Older keyboards might instead say , abbreviating the typewriter keyboard meaning 'Carriage-Return' which generated an electromechanical return to the left stop (CR code in ASCII-hex &0D;) and a line feed or move to the next line (LF code in ASCII-hex &0A;); in some applications these were independently used to draw text cell based displays on monitors or for printing on tractor-guided printers—which might also contain reverse motions/positioning code sequences allowing text-based output devices to achieve more sophisticated output. Many early computer games used such codes to draw a screen (e.g. Kingdom of Kroz), and word processing software would use this to produce printed effects such as bold, underline, and strikeout.
The term "whitespace" is based on the resulting appearance on ordinary paper. However they are coded inside an application, whitespace can be processed the same as any other character code and programs can do the proper action as defined for the context in which they occur.
Definition and ambiguity
The most common whitespace characters may be typed via the space bar or the tab key. Depending on context, a line-break generated by the return or enter key may be considered whitespace as well.
The table below lists the twenty-five characters defined as whitespace ("WSpace=Y", "WS") characters in the Unicode Character Database. Seventeen use a definition of whitespace consistent with the algorithm for bidirectional writing ("Bidirectional Character Type=WS") and are known as "Bidi-WS" characters. The remaining characters may also be used, but are not of this "Bidi" type.
Note: Depending on the browser and fonts used to view the following table, not all spaces may be displayed properly.
Non-breaking space: identical to U+0020, but not a point at which a line may be broken. HTML/XML named entity: , LaTeX:
ogham space mark
Used for interword separation in Ogham text. Normally a vertical line in vertical text or a horizontal line in horizontal text, but may also be a blank space in "stemless" fonts. Requires an Ogham font.
One-fifth (sometimes one-sixth) of an em wide. Recommended for use as a thousands separator for measures made with SI units. Unlike U+2002 to U+2008, its width may get adjusted in typesetting. HTML/XML named entity:  ; LaTeX: '\,'
Narrow no-break space. Similar in function to U+00A0 No-Break Space. When used with Mongolian, its width is usually one third of the normal space; in other context, its width sometimes resembles that of the Thin Space (U+2009).
MMSP. Used in mathematical formulae. Four-eighteenths of an em. In mathematical typography, the widths of spaces are usually given in integral multiples of an eighteenth of an em, and 4/18 em may be used in several situations, for example between the a and the + and between the + and the b in the expression a + b. HTML/XML named entity:
MVS. A narrow space character, used in Mongolian to cause the final two characters of a word to take on different shapes. It is no longer classified as space character (i.e. in Zs category) in Unicode 6.3.0, even though it was in previous versions of the standard.
ZWSP, zero-width space. Used to indicate word boundaries to text processing systems when using scripts that do not use explicit spacing. It is similar to the soft hyphen, with the difference that the latter is used to indicate syllable boundaries, and should display a visible hyphen when the line breaks at it. HTML/XML named entity: ​
zero width non-joiner
ZWNJ, zero-width non-joiner. When placed between two characters that would otherwise be connected, a ZWNJ causes them to be printed in their final and initial forms, respectively. HTML/XML named entity: ‌
zero width joiner
ZWJ, zero-width joiner. When placed between two characters that would otherwise not be connected, a ZWJ causes them to be printed in their connected forms. HTML/XML named entity: ‍
WJ, word joiner. Similar to U+200B, but not a point at which a line may be broken. HTML/XML named entity: ⁠
^ abcdefghijklmno Although considered to be a valid character in an internationalised domain name, in practice it is presented in more distinguishable Punycode so as to prevent homograph attacks where a third party masquerades as an already trusted party.
The Braille Patterns Unicode block contains ⠀BRAILLE PATTERN BLANK (HTML ⠀), a Braille pattern with no dots raised. Some fonts display the character as a fixed-width blank, however the Unicode standard explicitly states that it does not act as a space.
The Cambridge Z88 provided a special "exact space" (code point 160 aka 0xA0) (invokable by key shortcut +,) displayed as "..." by the operating system's display driver. It was therefore also known as "dot space" in conjunction with BBC BASIC.
Under code point 224 (0xE0) the computer also provided a special three-character-cells-wide SPACE symbol "SPC" (analogous to Unicode's single-cell-wide U+2420).
Whitespace and digital typography
Text editors, word processors, and desktop publishing software differ in how they represent whitespace on the screen, and how they represent spaces at the ends of lines longer than the screen or column width. In some cases, spaces are shown simply as blank space; in other cases they may be represented by an interpunct or other symbols. Many different characters (described below) could be used to produce spaces, and non-character functions (such as margins and tab settings) can also affect whitespace.
Variable-width general-purpose space
In computer character encodings, there is a normal general-purpose space (Unicode character U+0020) whose width will vary according to the design of the typeface. Typical values range from 1/5 em to 1/3 em (in digital typography an em is equal to the nominal size of the font, so for a 10-point font the space will probably be between 2 and 3.3 points). Sophisticated fonts may have differently sized spaces for bold, italic, and small-caps faces, and often compositors will manually adjust the width of the space depending on the size and prominence of the text.
In addition to this general-purpose space, it is possible to encode a space of a specific width. See the table below for a complete list.
Hair spaces around dashes
Em dashes used as parenthetical dividers, and en dashes when used as word joiners, are usually set continuous with the text. However, such a dash can optionally be surrounded with a hair space, U+200A, or thin space, U+2009. The hair space can be written in HTML by using the numeric character references  or  , or the named entity  , but is not universally supported in browsers yet, as of 2016.[update][which?] The thin space is named entity   and numeric references   or  . These spaces are much thinner than a normal space (except in a monospaced (non-proportional) font), with the hair space being the thinner of the two.
Normal space versus hair and thin spaces (as rendered by your browser)
In programming language syntax, spaces are frequently used to explicitly separate tokens. In most languages multiple whitespace characters are treated the same as a single whitespace character (outside of quoted strings); such languages are called free-form. In a few languages, including Haskell, occam, ABC, and Python, whitespace and indentation are used for syntactical purposes. In the satirical language called Whitespace, whitespace characters are the only valid characters for programming, while any other characters are ignored.
Excessive use of whitespace, especially trailing whitespace at the end of lines, is considered a nuisance. However correct use of whitespace can make the code easier to read and help group related logic.
Most languages only recognize ASCII characters as whitespace, or in some cases Unicode newlines as well, but not most of the characters listed above. The C language defines whitespace characters to be "space, horizontal tab, new-line, vertical tab, and form-feed". The HTTP network protocol requires different types of whitespace to be used in different parts of the protocol, such as: only the space character in the status line, CRLF at the end of a line, and "linear whitespace" in header values.
Command line user interfaces
In commands processed by command processors, e.g., in scripts and typed in, the space character can cause problems as it has two possible functions: as part of a command or parameter, or as a parameter or name separator. Ambiguity can be prevented either by prohibiting embedded spaces, or by enclosing a name with embedded spaces between quote characters.
Some markup languages, such as SGML, preserve whitespace as written.
Web markup languages such as XML and HTML treat whitespace characters specially, including space characters, for programmers' convenience. One or more space characters read by conforming display-time processors of those markup languages are collapsed to 0 or 1 space, depending on their semantic context. For example, double (or more) spaces within text are collapsed to a single space, and spaces which appear on either side of the "=" that separates an attribute name from its value have no effect on the interpretation of the document. Element end tags can contain trailing spaces, and empty-element tags in XML can contain spaces before the "/>". In these languages, unnecessary whitespace increases the file size, and so may slow network transfers. On the other hand, unnecessary whitespace can also inconspicuously mark code, similar to, but less obvious than comments in code. This can be desirable to prove an infringement of license or copyright that was committed by copying and pasting.
In XML attribute values, sequences of whitespace characters are treated as a single space when the document is read by a parser. Whitespace in XML element content is not changed in this way by the parser, but an application receiving information from the parser may choose to apply similar rules to element content. An XML document author can use the xml:space="preserve" attribute on an element to instruct the parser to discourage the downstream application from altering whitespace in that element's content.
In most HTML elements, a sequence of whitespace characters is treated as a single inter-word separator, which may manifest as a single space character when rendering text in a language that normally inserts such space between words. Conforming HTML renderers are required to apply a more literal treatment of whitespace within a few prescribed elements, such as the pre tag and any element for which CSS has been used to apply pre-like whitespace processing. In such elements, space characters will not be "collapsed" into inter-word separators.
In both XML and HTML, the non-breaking space character, along with other non-"standard" spaces, is not treated as collapsible "whitespace", so it is not subject to the rules above.
Such usage is similar to multiword file names written for operating systems and applications that are confused by embedded space codes—such file names instead use an underscore (_) as a word separator, as_in_this_phrase.
Another such symbol was ␢BLANK SYMBOL. This was used in the early years of computer programming when writing on coding forms. Keypunch operators immediately recognized the symbol as an "explicit space". It was used in BCDIC,EBCDIC, and ASCII-1963.