Tab key

Tab key on a standard keyboard (on upper left)

Tab ↹ Tab key (abbreviation of tabulator key or tabular key) on a keyboard is used to advance the cursor to the next tab stop.

History

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, called a tab stop. 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, often a first tab stop at 5 or 6 characters was used for this, far larger than the indentation used when typesetting.

Modern usage

In word processing and text editing the Tab key will often move the insertion point to the next tab stop in a table, or may insert the ASCII tab character or many space characters.

When filling out a computerized form, pressing Tab will move the cursor to the next 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 Windows, the Tab key will move the focus to every 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 OS X, this is an option called "Full Keyboard Access".

Tab may be used to complete a partially typed piece of text. For example, in some command-line interfaces, you may type the first few characters of a command or file-name, then press Tab. If there is no ambiguity about your intent, the rest of the characters will appear automatically. This usage is more common on Unix than Windows.

Tab characters

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. In C and many other programming languages the escape code \t can be used to put this character into a string constant. The horizontal tab is usually generated by the Tab key on a standard keyboard.

A vertical tab (VT) also exists and has ASCII decimal character code 11 (control+K or ^K), escape character \v.

In EBCDIC the code for HT is 5, and VT is 11 (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 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. For example, in Microsoft Word 2010, Insert > Table > 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.

Gopher menus use tab-separated values to indicate selectors.

TSV has also been cited in a modern approach to solving the programming fundamentalist debate regarding the use of tabs and spaces for code alignment called Elastic tabstops. Arguments have been made to replace the antiquated vertical tabs with columnar alignment in source code editors.

HTML

HTML represents the horizontal tab as 	 and 	 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 	 in HTML using <pre></pre> tags:

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.

The vertical tab is  but is not allowed in SGML; this includes XML 1.0 and HTML. In CSS, tabs are preserved in an element as shown above if the attribute white-space set to pre. CSS 3 defines tab-size property, which adjusts the number of spaces for the tab character from the default of 8. The latest version of WebKit supports the tab-size property. 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.

Unicode

The Unicode code points for the (horizontal) tab character, and the more rarely used vertical tab character are copied from ASCII:

• U+0009 <control-0009> (CHARACTER TABULATION, horizontal tabulation (HT), tab)
• U+000B <control-000B> (LINE TABULATION, vertical tabulation (VT))

The tab characters can be graphically represented by special symbols:

• U+2409 ␉ SYMBOL FOR HORIZONTAL TABULATION
• U+240B ␋ SYMBOL FOR VERTICAL TABULATION

Unicode also has characters for the symbols to represent or be printed on the tab key:

• 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

• Comma-separated values (CSV)
• Indent style

External links

• 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