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Incomputing, amodifier key is a special key (or combination) on acomputer keyboard that temporarily modifies the normal action of another key when pressed simultaneously. In general, pressing a modifier key on its own does nothing; therefore pressing any of the⇧Shift,Alt, orCtrl keys alone will not usually trigger computer action. A modifier key will commonly be used in a defined sequence of key presses in conjunction with another key, triggering a specific action. These sequences are calledkeyboard shortcuts.
For example, in mostkeyboard layouts the Shift key combination⇧ Shift+A will produce a capital letter "A" instead of the default lower-case letter "a" (unless inCaps Lock orShift lock mode). A combination ofAlt+F4 inMicrosoft Windows will trigger the shortcut for closing theactive window; in this instance, Alt is the modifier key. In contrast, pressing just⇧ Shift orAlt will probably do nothing unless assigned a specific function in a particular program (for example, activating input aids or the toolbar of the active window in Windows).
User interface expertJef Raskin coined the term "quasimode" to describe the state a computer enters into when a modifier key is pressed.
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The most common are:
The (Sun) Meta key, Windows key, (Apple) Cmd key, and the analogous "Amiga key" (A) onAmiga computers, are usually handled equivalently. Under the Linux operating system, the desktop environmentKDE calls this keyMeta, whileGNOME calls this key, neutrally,Super. This could be considered confusing, since the originalspace-cadet keyboard and theX Window System recognize a "Meta" modifier distinct from "Super".
TheZX Spectrum has a Symbol Shift key in addition to Caps Shift. This was used to access additional punctuation and keywords.
TheMSX computer keyboard, besides Shift and Control, also included two special modifier keys, Code and Graph. In some models, as in the BrazilianGradiente Expert, the Code and Graph keys are labelled "L Gra" and "R Gra" (Left and Right Graphics). They are used to select special graphic symbols and extended characters.
Likewise, theCommodore 64 and other Commodore computers had the Commodore key at the bottom left of the keyboard.
Compact keyboards, such as those used inlaptops, often have aFn key to save space by combining two functions that are normally on separate keys. On laptops, pressingFn plus one of the function keys, e.g., F2, often control hardware functions. Keyboards that lack a dedicated numeric keypad may mimic its functionality by combining the Fn key with other keys.
TheMITspace-cadet keyboard had additional Top and Front modifier keys. Combined with standard modifiers, it could enter as many as 8,000 different characters.
Specialist typesetting machines, and word processors such as theRedactron, sometimes used multiple modifier keys to trigger mode changes e.g. for emboldened text or justification changes. This approach gradually became obsolete after software based on commodity hardware and operating systems adopted theWIMP metaphor which provided drop-down menus etc.
Some non-English language keyboards have special keys to produceaccented modifications of the standard Latin-letter keys. In fact, the standard Britishkeyboard layout includes an accent key on the top-left corner to produce àèìòù, although this is a two step procedure, with the user pressing the accent key, releasing, then pressing the letter key. These kinds of keys are calleddead keys. TheAltGr modifier produces the áéíóú sequence, or in conjunction with the Shift key, ÁÉÍÓÚ. Keyboards of some languages simply include the accented characters on their own keys. Some keyboards also have aCompose key for typing accented and other special characters. By pressingCompose, and then two other keys, something similar to a combination of the glyphs of the two previous keys will appear on the screen.
It is possible to use a modifier key as a normal key.
In 2005 macOS appQuicksilver introduced a feature called 'Modifier-only Activation'. If a modifier key (Command) was pressed for a short duration (under 300ms) then released with no other key being pressed, this was taken as a 'trigger'.
In 2012 this kind of use of a Modifier key appeared in Apple's 'Dictation preferences' underOS X Mountain Lion, where Apple introduced options like 'Press Right Command Key twice' to launch dictation.
This functionality was further increased in macOS Sequoia (2024) with the addition of modifier only keys to be used alone to activate shortcuts such as 'Show Desktop, including Left Command, Left Option, Left Control, Left Shift, Right Command, Right Control, Right Option, Right Shift, fn. This effectively gives users 9 extra keys to activate shortcuts.
It is also possible to use (with some utility software) one key both as a normal key and as a modifier.
For example, you can use thespace bar both as a normal Space bar and as a Shift. Intuitively, it will act as a standard Space when you want a whitespace, and a Shift when you want it to act as a shift. I.e. when you simply press and release it, it is the usual space, but when you press other keys, sayX,Y andZ, while holding down thespace, then they will be treated as⇧ Shift plus X, Y and Z.
The above example is known as "SandS", standing for "Space and Shift" in Japan.[1] But any number of any combinations are possible.[2]
To press shift+space in the previous example, you need in addition to a space/shift dual role key, one of (a) another space/shift key, (b) a usual shift, or (c) a usual space key.