GNU Emacs Manual - Prefix Keymaps

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### Prefix Keymaps

A prefix key such as `C-x` or `ESC` has its own keymap, which holds the definition for the event that immediately follows that prefix.

The definition of a prefix key is usually the keymap to use for looking up the following event. The definition can also be a Lisp symbol whose function definition is the following keymap; the effect is the same, but it provides a command name for the prefix key that can be used as a description of what the prefix key is for. Thus, the binding of `C-x` is the symbol `Ctl-X-Prefix`, whose function definition is the keymap for `C-x` commands. The definitions of `C-c`, `C-x`, `C-h` and `ESC` as prefix keys appear in the global map, so these prefix keys are always available.

Aside from ordinary prefix keys, there is a fictitious "prefix key" which represents the menu bar; see section `Menu Bar' in The Emacs Lisp Reference Manual, for special information about menu bar key bindings. Mouse button events that invoke pop-up menus are also prefix keys; see section `Menu Keymaps' in The Emacs Lisp Reference Manual, for more details.

Some prefix keymaps are stored in variables with names:

- `ctl-x-map` is the variable name for the map used for characters that follow `C-x`.
- `help-map` is for characters that follow `C-h`.
- `esc-map` is for characters that follow `ESC`. Thus, all Meta characters are actually defined by this map.
- `ctl-x-4-map` is for characters that follow `C-x 4`.
- `mode-specific-map` is for characters that follow `C-c`.

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