The meta key is a modifier key on certain keyboards, specifically MIT and LISP machine keyboards and successors, such as the Knight keyboard, space-cadet keyboard (where it is labeled “META”), Symbolics keyboards (where it is labeled “META” or “Meta”), and on Sun Microsystems keyboards (where it is marked as a solid diamond “◆”).
On keyboards that lack a physical Meta key (or keys), its functionality may be invokable by other keys such as the Alt key, the Windows key, or user-configured keys. If no Meta key function is available, some software will accept alternate keybindings using, for example, the Esc key as a prefix key (as, for example, in Emacs).
On some systems the Meta modifier may be used in a manner similar to Macintosh's command key, but has been used in other ways, including as a user-configurable modifier in the X Window System, or in combination with other modifier keys such as Hyper and Super, also seen on the classic LISP keyboards and supported by windows systems such as X.
One notable difference from the other keys, dating to the Knight keyboard, is that the meta key was outside the control key, instead of inside the control key (as the Alt key is usually placed). This results in modifier keys for Emacs keybindings that were designed for Lisp machine keyboards being swapped on standard keyboards.
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