Tab key (abbreviation of tabulator key[1] or tabular key[2]) on a keyboard is used to advance the cursor to the next tab stop.
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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.
In modern applications, the Tab key rarely has the effect of inserting the Tab character. In word processor applications, the Tab key typically moves the cursor to the next tab stop. In most other graphical applications, the Tab key will shift the focus to the next control or widget.
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). The EBCDIC code for HT is 5. The VT is 11 or hex 0B, 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.
It is unclear why the 8-character horizontal tab size was chosen, since 5 characters, a half inch in a typical printer at that time, was much more popular as a paragraph indentation. The number 8 may have been chosen to match early Fortran conventions, where the statement text started after the line number and continuation character. It may have been chosen as the smallest size that would fit numbers typically printed in a table.
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.
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.
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 pre.
Here is an example showing the use of 	 with <pre></pre> tags. If you write in HTML:
These 2 lines are tabbed: 2009	This line uses a tab space. 	This line also uses a tab space. This line does not use a tab space.
The result would be something like this:
These 2 lines are tabbed: 2009 This line uses a tab space. This line also uses a tab space. This line does not use a tab space.
The vertical tab is  but is not allowed in SGML, including HTML and XML 1.0.
The issue is of concern in the use of CSS, which asserts that in an element where white space is to be preserved:
... 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.
The Opera web browser supports the -o-tab-size CSS property, which adjusts the default number of spaces for the tab character. [3]
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