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The control key, often labeled “Ctrl” or “Ctl” on a computer keyboard, is a special type of key called a modifier key that is intended to be pressed down in combination with one or more other keys. Its exact function depends on the software and operating system being used, but it most often performs common functions like saving documents, printing, and copy/paste procedures. The control key’s name comes from its original purpose in teletype machines, where it was used to control aspects of the machine’s operation.
Modifier keys, which can include shift, alt, control, and other keys depending on keyboard layout, are unique in that they are seldom used by themselves. These types of keys are instead meant to be used in conjunction with other keys on the keyboard in a technique known as a key combination. Most computer users are familiar with the shift key combination, in which the shift key is used in combination with another key to produce upper case characters. The control key also modifies the usual output of keys, but generally changes the output to a command or function rather than a special character. Holding control while pressing the “Q” key (Ctrl+Q), for example, will quit an application, while Ctrl+P can be used to print a document or webpage.
The precise impact that a control key combination has depends on the software and operating system being used. Pressing Ctrl+R in many web browsers will reload a webpage, while the same key combination in a word processing program often aligns a paragraph to the right. Common functions like saving, printing, and copy/paste are generally the same across many types of programs, while other functions may be specific to a certain program or even individual elements of that program. More complex combinations involving as many as four keys are possible; in many cases, users can customize these combinations to perform the operations they desire. In some operating systems, the control key’s usual functions are replaced by a different modifier key, while the control key itself is used for a separate purpose. Users can easily find some of these combinations by consulting the manual or searching online for "keyboard shortcuts" and the name of the program.
Like many other elements of the computer keyboard, the control key is a vestige of earlier technology that has been updated and adapted to remain relevant in modern computing environments. Originally found on teletype machines, the control key produced special key codes that were not printed or displayed on the screen, but controlled some aspect of the machine’s operation. Some examples of these codes, which were known as control characters, include Ctrl+J for a line feed, Ctrl+H for backspace, and even Ctrl+G to ring a bell in the terminal.