Modifier key

In computing, a modifier key is a special key on a computer keyboard that modifies the normal action of another key when the two are pressed in combination.

For example, <Alt> + <F4> in Microsoft Windows will automatically close the program in an active window. In contrast, pressing just <F4> will probably do nothing unless assigned a specific function in a particular program. By themselves, modifier keys usually do nothing.

Common modifier keys

The most common modifier keys, as used by PCs, are:

<Shift> – Shift key
<Ctrl> – Control key
<Alt> – Alt key (Alternative key)
<Alt Gr> – Alt Gr key (Alternative Graphics key) – used to obtain special symbols from the keyboard. If a key has a third symbol on it (sometimes in a different colour or on the vertical edge of the key), then <Alt Gr> will elicit that symbol. Under Microsoft Windows, <Alt Gr> is equivalent to <Ctrl><Alt>.

<Shift>, of course, is the oldest in terms of keyboards in general as it originates from typewriters and is usually used to make uppercase letters when typing.

Accented characters

Some foreign language keyboards have special keys to produce accented modifications of the standard letter keys. In fact, the standard British keyboard layout includes an accent key on the top-left corner to produce ��, although this is a two step procedure, press the accent key and release, then the letter key. The Alt Gr modifier produces the �� sequence, or in conjunction with the Shift key, ��. Some foreign language keyboards simply include the accented characters on their own keys.

Other modifier keys

Apple Macintosh computers also have the special <Command> key with the Apple Computer logo and the place of interest sign (Unicode code point U+2318, or ⌘). <Alt> is also replaced with the <Option> key (⌥); however modern Apple keyboards label this key with both "option" and "alt". See also the Jargon file on feature key for details on the history of this sign: http://www.catb.org/~esr/jargon/html/F/feature-key.html

Sun Microsystems keyboards have two <Meta> keys, which look like a square rotated 45 degrees (like a diamond shape; Unicode code point U+25C7 or ◇).

Amiga computers has two <Amiga> keys with the Amiga logo on them. They are mostly used in the same way as <Alt> is on the PC.