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 with a moveable lever stop for every position across the page. Initially these were set by hand, but later tab set and tab clear keys were added. When the tab key was depressed, the carriage advanced to the next tab stop. These were set to correspond to the particular column locations of the table, hence tab, being worked on. The tab mechanism also came into its own as a rapid and consistent way of uniformly indenting the first line of each paragraph.
Tab characters 
Several tab characters are included as ASCII control characters, used for text alignment. The most known and common tab is a horizontal tab (HT), which in ASCII has the decimal character code of 9, and may be referred to as control+I or ^I. A vertical tab (VT) also exists and has ASCII decimal character code 11 (control+K or ^K). In EBCDIC the code for HT is 5, and VT is 11 (the same as in ASCII). The horizontal tab is usually generated by the Tab key on a standard keyboard.
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 loop of mylar or other tape the length of a page with holes punched in it to indicate the tab stops. Initially these were manually set up to match the preprinted forms that were loaded into the printer. Later, the intention was to have the machine be pre-programmed, by using other control characters to set and clear the stops: ISO 6429 includes the codes 136 (Horizontal Tabulation Set), 137 (Horizontal Tabulation with Justification) and 138 (Vertical Tabulation Set).
In practice, settable tab stops were rather quickly replaced with fixed tab stops, de facto standardized at every multiple of 8 characters horizontally, and every 6 lines vertically (typically one inch vertically). A printing program could easily send the necessary spaces or line feeds to move to any position wanted on a form, and this was far more reliable than the modal and non-standard methods of setting tab stops. Tab characters simply became a form of data compression.
A common horizontal tab size of eight characters evolved, despite five characters being half an inch and the typical paragraph indentation of the time, because as a power of two it was easier to calculate in binary for the limited digital electronics available.
Tab-separated values (TSV) 
Tab-separated values (TSV) are a common de facto standard 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.[further explanation needed]
The TSV convention for exporting data may be compared to the alternative comma-separated values (CSV) convention.
Gopher menus use tab-separated values to indicate selectors.
Tabs in HTML 
HTML represents the horizontal tab as
 but as with all whitespace characters this does not allow actual insertion of tabs into the page except inside
<pre></pre> tags or elements with CSS attribute
white-space set to
Here is an example showing the use of
in HTML using
These 2 lines are tabbed: 2009	This line uses a tab. 	This line also uses a tab. This line does not use a tab.
The result would be something like this:
These 2 lines are tabbed: 2009 This line uses a tab. This line also uses a tab. This line does not use a tab.
... 2. All tabs (U+0009) are rendered as a horizontal shift that lines up the start edge of the next glyph with the next tab stop. Tab stops occur at points that are multiples of 8 times the width of a space (U+0020) rendered in the block's font from the block's starting content edge.
CSS 3 defines
tab-size property, which adjusts the default number of spaces for the tab character. The latest version of WebKit supports it. The Opera web browser supports the
-o-tab-size CSS property, the Firefox web browser supports the
-moz-tab-size CSS property with the same meaning.
- U+21B9 ↹ leftwards arrow to bar over rightwards arrow to bar ("tab with shift tab")
- U+21C6 ⇆ leftwards arrow over rightwards arrow
- U+21E4 ⇤ leftwards arrow to bar ("leftward tab")
- U+21E5 ⇥ rightwards arrow to bar ("rightward tab")
See also 
- "Underwood Portable Typewriter Gallery". Retrieved 2011-04-09.
- "Instructions for Using the Underwood Typewriter". Retrieved 2011-04-09.
- See Character encodings in HTML#HTML character references
- "CSS 3 tab-size property W3 specification".
- "CSS tab-size property on Mozilla Developer Network".
- http://www.ecma-international.org/publications/standards/Ecma-262.htm Section 7.8.4 String Literals
- "Arrows – Unicode". Retrieved 2013-05-11.
- Tabs versus Spaces: An Eternal Holy War by Jamie Zawinski
- Why I prefer no tabs in source code by Adam Spiers
- Why I love having tabs in source code
- Elastic tabstops – the solution to the tabs-versus-spaces issue