View source for Tab key

{{short description|Key on a keyboard for tabulation}}
{{More citations needed|date=February 2023}}
[[Image:Keyboard-left keys.jpg|thumb|Tab key (top left) on a standard [[Windows keyboard]]]]
The '''tab key''' {{keypress|Tab}} (abbreviation of '''tabulator key'''<ref>{{cite web|url=http://machinesoflovinggrace.com/ptf/UnderwoodPortableGallery.html|title=Underwood Portable Typewriter Gallery|access-date=2011-04-09|url-status=dead|archive-url=https://web.archive.org/web/20110222053316/http://machinesoflovinggrace.com/ptf/UnderwoodPortableGallery.html|archive-date=2011-02-22}}</ref> or '''tabular key''')<ref>{{cite web|url=http://www.machinesoflovinggrace.com/manuals/Underwood3-5manual.pdf|title=Instructions for Using the Underwood Typewriter|access-date=2011-04-09}}</ref> on a [[alphanumeric keyboard|keyboard]] is used to advance the cursor to the next [[tab stop]].

== History ==

[[File:Friden 2201 tab rack.jpg|thumb|The tab rack from a [[Flexowriter]] model 2201. On this machine, the tab-rack is removable for easy reconfiguration.]]
The word ''tab'' derives from the word ''tabulate'', which means "to arrange data in a tabular, or table, form". When a person wanted to type a table (of numbers or text) on a [[typewriter]], there was a lot of time-consuming and repetitive use of the [[space bar]] and [[backspace]] key. To simplify this, a horizontal bar was placed in the mechanism called the tabulator rack. Pressing the tab key would advance the carriage to the next [[tab stop|tabulator stop]]. The original tabulator stops were adjustable clips that could be arranged by the user on the tabulator rack. Fredric Hillard filed a patent application for such a mechanism in 1900.<ref>Fredric W. Hillard, Type-writing machine, {{US patent|720520}}, granted Feb. 10, 1903.</ref>

The tab mechanism came into its own as a rapid and consistent way of uniformly [[Indentation (typesetting)|indenting]] the first line of each paragraph. Often a first tab stop at 5 or 6 characters was used for this, far larger than the indentation used when typesetting. For numeric data, however, the logical place for the tab stop is the location of the least significant digit. Tabbing to this location and then pressing the [[backspace]] key to back up to the position of the first digit is practical but slow. Various schemes for numeric tabs were proposed. For example, in 1903, Harry Dukes and William Clayton filed for a patent on a tab mechanism with multiple tab keys numbered 1, 10, 100, etc. Pressing 1 was a simple tab. Pressing 10 advanced to the space before the tab, pressing 100 advanced to the position 2 spaces before the tab.<ref>Harry S. Dukes and William H. Clayton, Tabulating mechanism for type-writing machines, {{US patent|908221}}, granted Dec. 29, 1908.</ref>

Initially tab stops were set by adding and removing clips from the tab rack, but Edward Hess working for the [[Royal Typewriter Company]] filed for a patent in 1904 covering a system where the tab stops were permanently mounted on the tab bar. To set or reset a tab for a particular column, the tab stop for that column was simply rotated in or out of engagement.<ref>Edward B. Hess, Writing Machine, {{US patent|931305}}, granted Aug. 17. 1909.</ref> In 1940, James Koca filed for a patent on a mechanism allowing the tab stops for each column to be set and cleared from the keyboard, eliminating the need for the typist to bend over the back of the machine to directly manipulate the tab rack.<ref>James F. Koca, Typewriter stop mechanism, {{US patent|2358517}}, granted Sep. 19, 1944.</ref> These keys, if present, are typically labeled '''tab set''' and '''tab clear'''.

== Modern usage ==

In [[Word processor|word processing]] and [[Text editor|text editing]], the Tab key will move the insertion point to the next tab stop in a table, insert the ASCII tab character, or insert multiple space characters (depending on the program used).

When filling out a computerized form, pressing Tab will sometimes move the cursor to the next [[Text field|field]] (and Shift-Tab will move the cursor to the previous field), eliminating the need to use a mouse to click in an adjacent field.

In many graphical applications, especially on [[Microsoft Windows|Windows]], the Tab key will move the [[Focus (computing)|focus]] to ''every'' [[GUI widget|control or widget]] such as buttons so that the user interface can be used without a mouse at all (this was part of the [[IBM Common User Access]] design). On [[macOS]], this is an option called "Full Keyboard Access".

A feature called [[Command-line completion|tab completion]] can be used to complete a partially typed piece of text. For example, in some [[command-line interface]]s, the user may type the first few characters of a command or file-name, then press Tab. If there is no ambiguity about the intent, the rest of the characters will appear automatically. On some systems, even if the input is ambiguous, tab completion may give the user a list of possible options to select from. Tab completion is more common on [[Linux]], [[Unix]], and [[Unix-like]] systems than [[Microsoft Windows|Windows]].

In [[Personal computer|PC]] [[video game]]s, the Tab key is very often used to show scores in multiplayer games. For single player games it is also used to show the [[Virtual world|in-game world map]] or the player's inventory, as well as other useful information.

In [[software engineering]], when developing computer programs or storing and manipulating data in files, the Tab character is often used for indentation purposes to help guide the flow of reading and add semantic structure to the code or data. Some programmers and programming languages prefer the usage of multiple whitespace characters instead for that purpose. Because of this, many text editors have an option that makes the tab key insert the number of whitespace characters that a tab character is set to be equivalent to (by default mostly four).

== Tab characters ==
The most known and common tab is a ''horizontal tabulation'' (HT) or ''character tabulation'', which in [[ASCII]] has the decimal character code of 9, and may be referred to as {{key press|Ctrl|I}} or {{mono|^I}}. In [[C (programming language)|C]] and many other [[programming language]]s the [[escape sequence]] {{code|\t}} can be used to put this character into a [[string literal]]. The horizontal tab is usually inserted when the Tab key on a standard keyboard is pressed.

A ''vertical tabulation'' (VT) also exists and has ASCII decimal character code 11 ({{key press|Ctrl|K}} or {{mono|^K}}), escape character {{code|\v}}.

In [[EBCDIC]] the code for HT is 5. VT is 11 (coincidentally the same as in ASCII).

Originally, printer mechanisms used mechanical tab stops to indicate where the tabs went. This was done horizontally with movable metal prongs in a row, and vertically with a [[carriage control tape|loop of mylar or other tape the length of a page with holes punched in it]] to indicate the tab stops. These were manually set to match the pre-printed forms that were loaded into the printer. In practice, settable tab stops were rather quickly replaced with fixed tab stops, ''de facto'' standardized at every multiple of eight characters horizontally, and every six lines vertically. A printing program could send zero or more tabs to get to the closest tab stop above and left of where it wanted to print, then send line feeds and spaces to get to the final location. Tab characters simply became a form of data compression.

Despite the fact that five characters were the typical paragraph indentation on typewriters at that time, the horizontal tab size of eight evolved because as a power of two it was easier to calculate with the limited digital electronics available.{{citation needed|date=December 2023}} Using this size tab to indent code results in much white space on the left, so most text editors for code, such as [[integrated development environment|IDE]]s, allow the size of the tab to be changed, and some (in particular on Windows) default to four instead of eight. Disagreements between programmers about [[indentation style|what size tabs are correct]], and whether to use tabs at all, are common.<ref>{{Cite web|title=Tabs versus Spaces|url=https://www.jwz.org/doc/tabs-vs-spaces.html|access-date=2020-06-02|website=www.jwz.org}}</ref> Modern text editors usually have the Tab key insert the user-defined indentation and may use heuristics to adapt this behavior to existing files.

[[ISO 6429]] includes the codes 136 (Horizontal Tabulation Set), 137 (Horizontal Tabulation with Justification) and 138 (Vertical Tabulation Set) that were intended to allow the program to set and clear tab stops.{{dubious|editor's confusion about HTS|date=April 2024}} This is rarely used or supported.

=== Tab-separated values ===
[[Tab-separated values]] (TSV) are used for exporting and importing database or spreadsheet field values. Text divided into fields delimited by tabs can often be pasted into a [[word processor]] and formatted into a table with a single command. For example, in Microsoft Word 2010, Insert &gt; Table &gt; Convert Text to Table... is the necessary command, producing a dialog where the user selects further details.

The TSV convention for exporting data may be compared to the alternative [[comma-separated values]] (CSV) convention (that may be using semicolons instead of commas), and both are easily converted into each other.

[[Gopher (protocol)|Gopher menus]] use tab-separated values to indicate selectors.

TSV has also been cited in a modern approach to solving the programming debate regarding the use of tabs and spaces for code alignment called [[elastic tabstops]].<ref>{{cite web|url=http://nickgravgaard.com/elastic-tabstops/|title=Elastic tabstops - a better way to indent and align code|first=Nick|last=Gravgaard|website=nickgravgaard.com|access-date=23 March 2018}}</ref> This idea uses a scheme called tab-separated columns (TSC) rather than the similar tab-separated values (TSV).

=== HTML ===

In [[HTML]] the horizontal tab is coded using {{code|&#9;}} or {{code|&Tab;}}<ref>See [[Character encodings in HTML#HTML character references]]</ref><ref name="W3CCharacterRef">{{cite web|url=http://dev.w3.org/html5/html-author/charref|title=Character Entity Reference Chart|website=dev.w3.org|access-date=23 March 2018}}</ref> but as with all whitespace characters in HTML, this will be displayed as a single space except inside {{tag|pre|open}}, {{tag|code|open}} tags (or other elements with [[Cascading Style Sheets|CSS]] attribute {{code|white-space}} set to {{code|pre}}).

Here is an example:

<blockquote>
;HTML:
<syntaxhighlight lang="html">
&Tab;This line begins with a single tab.
Here&Tab;are&Tab;some&Tab;more&Tab;tab&Tab;characters&Tab;!
T.......T.......T.......T.......T.......T.......T.......T
</syntaxhighlight>
;Inside {{tag|pre|open}}:
{{not a typo|<pre>
&Tab;This line begins with a single tab.
Here&Tab;are&Tab;some&Tab;more&Tab;tab&Tab;characters&Tab;!
T.......T.......T.......T.......T.......T.......T.......T
</pre>}}
;Outside {{tag|pre|open}}:
<div style="font-family: monospace, monospace;">
{{not a typo|&Tab;This line begins with a single tab.}}
{{not a typo|Here&Tab;are&Tab;some&Tab;more&Tab;tab&Tab;characters&Tab;!}}
T.......T.......T.......T.......T.......T.......T.......T
</div></blockquote>

Notice that the tab at the start of a line is removed outside {{tag|pre|open}}, the eight-character spacing, and how a tab stop is skipped if there are more than eight characters since last one.

CSS3 defines {{code|tab-size}} property, which adjusts the number of spaces for the tab character from the default of eight.<ref>{{cite web|url=http://dev.w3.org/csswg/css3-text/#tab-size-property|title=CSS 3 tab-size property W3 specification}}</ref> The latest version of [[WebKit]] supports the {{code|tab-size}} property. The [[Opera (web browser)|Opera]] web browser supports the {{code|-o-tab-size}} CSS property, the [[Firefox]] web browser supports the {{code|-moz-tab-size}} CSS property with the same meaning.<ref>{{cite web|url=https://developer.mozilla.org/en-US/docs/CSS/tab-size|title=CSS tab-size property on Mozilla Developer Network|date=21 February 2023 }}</ref>

The vertical tab is {{code|&#xB;}} but is not allowed in [[SGML]]{{Citation needed|date=March 2013}}; this includes [[XML]] 1.0<ref>{{cite web|title=Extensible Markup Language (XML) 1.0|edition=5th|publisher=[[W3C]]|date=26 November 2008|url=https://www.w3.org/TR/xml/#charsets|at=sec. "Charsets"}}</ref> and HTML.

=== Unicode ===
The [[Unicode]] code points for the (horizontal) tab character, and the more rarely used vertical tab character are copied from [[ASCII]]:<ref>{{cite web| title = C0 Controls and Basic Latin | url = https://www.unicode.org/charts/PDF/U0000.pdf| access-date = 2013-12-13}}</ref>

* <big>{{unichar|0009|CHARACTER TABULATION}}</big> (CHARACTER TABULATION, horizontal tabulation (HT), tab)
* <big>{{unichar|000B|LINE TABULATION}}</big> (LINE TABULATION, vertical tabulation (VT))

The tab characters can be graphically represented by [[Control Pictures|special symbols]]:
* <big>{{unichar|2409|SYMBOL FOR HORIZONTAL TABULATION}}</big>
* <big>{{unichar|240B|SYMBOL FOR VERTICAL TABULATION}}</big>

Unicode also has characters for the symbols to represent or be printed on the tab key:<ref name="Unicode Arrows">{{cite web| title = Arrows – Unicode| url = https://www.unicode.org/charts/PDF/U2190.pdf| access-date = 2013-12-13}}</ref>

* <big>{{unichar|21B9|LEFTWARDS ARROW TO BAR OVER RIGHTWARDS ARROW TO BAR}}</big> (tab with shift tab)
* <big>{{unichar|21C6|LEFTWARDS ARROW OVER RIGHTWARDS ARROW}}</big>
* <big>{{unichar|21E4|LEFTWARDS ARROW TO BAR}}</big> (leftward tab)
* <big>{{unichar|21E5|RIGHTWARDS ARROW TO BAR}}</big> (rightward tab)

Unicode provides additional characters for tab symbols with triangle-headed arrow:<ref name="Unicode Miscellaneous Symbols and Arrows">{{cite web| title = Miscellaneous Symbols and Arrows – Unicode| url = https://www.unicode.org/charts/PDF/U2B00.pdf| access-date = 2023-08-31}}</ref>

* <big>{{unichar|2B70|LEFTWARDS TRIANGLE-HEADED ARROW TO BAR}}</big> (left tab)
* <big>{{unichar|2B71|UPWARDS TRIANGLE-HEADED ARROW TO BAR}}</big> (up tab)
* <big>{{unichar|2B72|RIGHTWARDS TRIANGLE-HEADED ARROW TO BAR}}</big> (right tab)
* <big>{{unichar|2B73|DOWNWARDS TRIANGLE-HEADED ARROW TO BAR}}</big> (down tab)
* <big>{{unichar|2B7E|HORIZONTAL TAB KEY}}</big>
* <big>{{unichar|2B7F|VERTICAL TAB KEY}}</big>

== See also ==
* [[Comma-separated values]] (CSV)
* [[Indentation style]]

== References ==
<references/>

{{Keyboard}}
{{keyboard keys}}

[[Category:Computer keys]]
[[Category:Control characters]]
[[Category:Whitespace]]