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Class of key on a computer keyboard

In computing, a modifier key is a special key (or combination) on a computer keyboard that temporarily modifies the normal action of another key when pressed together. By themselves, modifier keys usually do nothing; that is, pressing any of the , , or keys alone does not (generally) trigger any action from the computer. For example, in most keyboard layouts the Shift key combination will produce a capital letter "A" instead of the default lower case letter "a" (unless in Caps Lock or Shift lock mode). A combination of in Microsoft Windows will close the active window; in this instance, Alt is the modifier key. In contrast, pressing just or 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 expert Jef Raskin coined the term "quasimode" to describe the state a computer enters into when a modifier key is pressed.

Some non English language keyboards have special keys to produce accented modifications of the standard Latin letter keys. In fact, the standard British keyboard 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 called dead keys. The AltGr 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 a Compose key for typing accented and other special characters. By pressing , 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 Quicksilver (software) 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' under OS X Mountain Lion, where Apple introduced options like 'Press Right Command Key twice' to launch dictation.

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 the 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, say , and , while holding down the , then they will be treated as plus X, Y and Z. The above example is known as "SandS", standing for "Space and Shift" in Japan. But any number of any combinations are possible. 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.