Control key

A Control key (marked "Ctrl") on a Windows keyboard

In computing, a Control key is a modifier key which, when pressed in conjunction with another key, will perform a special operation (for example, Control-Alt-Delete); similar to the Shift key, the Control key rarely performs any function when pressed by itself. The control key is located on or near the bottom left side of most keyboards, with many featuring an additional one at the bottom right. It is usually labeled Ctrl, but sometimes Control or Ctl is seen, and it can be graphically represented as an “up arrowhead” (U+2303, ⌃), or simulated with a caret (^).

History

Control key on an Apple wireless keyboard

On teletypewriters and early keyboards, holding down the Control key while pressing another key zeroed the leftmost 2 bits of the 7 bits in the generated ASCII character. This allowed the operator to produce the first 32 characters in the ASCII table. These are non-printing characters that signal the computer to control where the next character will be placed on the display device, eject a printed page or erase the screen, ring the terminal bell, or some other operation. Aptly, these characters are also called control characters. In German keyboards, it is called a Strg key.

Using the Control key with either lowercase c or uppercase C will generate the same ASCII code on a teletypewriter because holding down the control key grounds (zeros the voltage on) the 2 wires used to carry the leftmost 2 bits from the keyboard. In modern computers, the interpretation of keypresses is generally left to the software. Modern keyboards distinguish each physical key from every other and report all keypresses and releases to the controlling software. This additional flexibility is not often taken advantage of and it usually does not matter, for example, whether the control key is pressed in conjunction with an upper or a lower case character.

When the original purpose of the ASCII control characters became either obsolete or seldom used, later software appropriated the Control key combinations for other purposes.

Because on older keyboards the Control key was located on the left of the keyboard, some people remap the keys to exchange Control and Caps Lock, finding the traditional location more ergonomic for using programs benefiting from use of the Control key. This is sometimes called a "Unix" layout. Keyboards from Sun Microsystems came in two layouts; Unix and PC-style, with the Unix layout having the traditional placing of the Control key and other keys. The OLPC XO-1 keyboard also has the Control key in this location. Keyboards for different computer systems with this placement of the Control key are also sold as extra peripherals, see for example the Happy Hacking Keyboard. After the Mac OS became a Unix based OS, a keyboard-preference setting was added specifically to address this need.

Others leave the control key in the lower-left corner of the keyboard, and press it using the side of their palm. The choice of location for the control key often comes down to the typist's hand shape and posture.

Notation

There are several common notations for pressing the Control key in conjunction with another key. Each notation below means press and hold Control while pressing the x key:

^X	Traditional notation
C-x	Emacs notation
CTRL-X	Old Microsoft notation
Ctrl+X	New Microsoft notation
Ctrl/X	OpenVMS notation

Mac OS and Mac OS X uses the Traditional notation in menus.

Examples

Different application programs, user interfaces, and operating systems use the various control key combinations for different purposes.

Key combination	Microsoft Windows/KDE/GNOME	Unix (command line and programs using readline)	Emacs (if different from Unix command line)
Ctrl+A	Select all	Beginning of line
Ctrl+B	Bold	Backward one character
Ctrl+C	Copy (can also be used as an alternative to Ctrl+Break to terminate an application)	terminate application	Compound command
Ctrl+D	Font Window (Word Processing); Add to bookmarks (Browsers)	Forward delete, or if line is empty, end of input (traditional Unix)	Forward delete
Ctrl+E	Center Alignment (Word Processing)	End of line
Ctrl+F	Find (usually a small piece of text in a larger document)	Forward one character
Ctrl+G	Go To (Line number)	Bell	Quit - aborts current operation
Ctrl+H	Replace, or History in browsers	Delete previous character	Help key
Ctrl+I	Italic (sometimes incremental search)	Command line completion	Same as Tab key
Ctrl+J	Justify	Line feed (LFD)	LFD (to evaluate Lisp expressions)
Ctrl+K	Insert Hyperlink (Word 2007)	Cut ("Kill") text between cursor and end of line
Ctrl+L	Create List, Left align (word processing)	Clear screen	Redraw window/terminal, and recenter view around current line
Ctrl+M	Decrease margin by 1/2 inch (Microsoft Word)	Same as Enter key
Ctrl+N	New (window, document, etc.)	Next line (in history)	Next line
Ctrl+O	Open	Flush output	Insert ("open") new line
Ctrl+P	Print	Previous line (in history)	Previous line
Ctrl+Q	Quit application	Resume transmission	Literal insert
Ctrl+R	Refresh page, Right align (word processing)	Search backwards in history	Search backwards
Ctrl+S	Save	Pause transmission	Search forward
Ctrl+T	Open New Tab	Transpose characters
Ctrl+U	Underline	Delete text between beginning of line and cursor	Prefix numerical argument to next command
Ctrl+V	Paste	Literal insert	Page down
Ctrl+W	Close window or tab	Delete previous word	Cut
Ctrl+X	Cut	Compound command
Ctrl+Y	Redo (sometimes Ctrl+Shift+Z is used for this)	Paste
Ctrl+Z	Undo	Suspend program	Iconify window
Ctrl+[	Decrease font size	Same as Esc key	Same as Modifier key (Alt key)
Ctrl+End	Bottom (end of document or window)	undefined or rarely used	Bottom (end of text buffer)
Ctrl+Home	Top (start of document or window)	undefined or rarely used	Top (start of text buffer)
Ctrl+Ins	Copy	undefined or rarely used	Copy
Ctrl+PgDn	Next tab	undefined or rarely used	Scroll window to the right
Ctrl+PgUp	Previous tab	undefined or rarely used	Scroll window to the left
Ctrl+Tab	Next window or tab	undefined or rarely used
Ctrl+Shift+Tab	Previous window or tab	undefined or rarely used
Ctrl + ←	Previous word	undefined or rarely used	Previous word
Ctrl + →	Next word	undefined or rarely used	Next word
Ctrl+Delete	Delete Next word	undefined or rarely used	Delete Next word
Ctrl+Backspace	Delete Previous word	undefined or rarely used	Delete Previous word
Ctrl+Alt+Backspace	undefined or rarely used	Restart X11	undefined or rarely used
Ctrl + Alt + ↑	Rotate Screen Upside down	unknown	unknown
Ctrl + Alt + ↓	Rotate Screen Right side up	unknown	unknown
Ctrl + Alt + ←	Rotate Screen left	unknown	unknown
Ctrl + Alt + →	Rotate Screen right	unknown	unknown
Ctrl+Shift+Esc	Task Manager	unknown	unknown
Ctrl+Alt+Delete	Windows Security/Task Manager	Reboot (when on the console)	undefined or rarely used

Similar concepts

Generally, the Command key, labeled with the ⌘ symbol on Apple Macintosh computers, performs the equivalent functions in Mac OS X and Mac OS applications (for example, ⌘C copies, while ⌘P prints; the same holds for saving, cutting, and pasting).

Macintoshes also have a Control key, but it has different functionality. The original Apple mouse design reduced complexity by only offering one button. As the interface developed, Contextual Menus were offered to access extra options. Another button was needed to access these. On Unix and Windows, the user had other mouse buttons to use. On Mac OS, the Control key was used to invoke a "right-click". Apple calls this a "secondary click" as left-handers can choose which side this button is on.

• It is mostly used as a modifier key for key-combinations.
• When pressing Control and clicking the mouse-button, you will get a contextual menu. This is a compatibility feature for users with one-button mice; users with two-button mice just use the right mouse-button, with no modifiers.
• It is used in the command line interface with programs made for that interface.
• In Quickbooks, the Control Key is used to validate login credentials during OLSU 1016 and OLSU 1013 errors. Keeping it depressed while sending information to the Quickbooks servers fixes the errors.
• Under Mac OS X, the Control key allows the use of Emacs-style key combinations in most text entry fields. For example, Ctrl-A moves the caret to the start of the paragraph, Ctrl-L vertically centers the current line in the editing field, Ctrl-K cuts text to the end of the line to a kill ring separate from the normal clipboard, etc.

See also

• Alt key
• Break key
• Control characters
• Control-Alt-Delete
• Function key
• Keyboard layout
• Modifier key