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(elisp)Prefix Keys

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Prefix Keys
===========

   A "prefix key" is a key sequence whose binding is a keymap.  The
keymap defines what to do with key sequences that extend the prefix key.
For example, `C-x' is a prefix key, and it uses a keymap that is also
stored in the variable `ctl-x-map'.  This keymap defines bindings for
key sequences starting with `C-x'.

   Some of the standard Emacs prefix keys use keymaps that are also
found in Lisp variables:

   * `esc-map' is the global keymap for the <ESC> prefix key.  Thus,
     the global definitions of all meta characters are actually found
     here.  This map is also the function definition of `ESC-prefix'.

   * `help-map' is the global keymap for the `C-h' prefix key.

   * `mode-specific-map' is the global keymap for the prefix key `C-c'.
     This map is actually global, not mode-specific, but its name
     provides useful information about `C-c' in the output of `C-h b'
     (`display-bindings'), since the main use of this prefix key is for
     mode-specific bindings.

   * `ctl-x-map' is the global keymap used for the `C-x' prefix key.
     This map is found via the function cell of the symbol
     `Control-X-prefix'.

   * `mule-keymap' is the global keymap used for the `C-x <RET>' prefix
     key.

   * `ctl-x-4-map' is the global keymap used for the `C-x 4' prefix key.

   * `ctl-x-5-map' is the global keymap used for the `C-x 5' prefix key.

   * `2C-mode-map' is the global keymap used for the `C-x 6' prefix key.

   * `vc-prefix-map' is the global keymap used for the `C-x v' prefix
     key.

   * `facemenu-keymap' is the global keymap used for the `M-g' prefix
     key.

   * The other Emacs prefix keys are `C-x @', `C-x a i', `C-x <ESC>'
     and `<ESC> <ESC>'.  They use keymaps that have no special names.

   The keymap binding of a prefix key is used for looking up the event
that follows the prefix key.  (It may instead be a symbol whose function
definition is a keymap.  The effect is the same, but the symbol serves
as a name for the prefix key.)  Thus, the binding of `C-x' is the
symbol `Control-X-prefix', whose function cell holds the keymap for
`C-x' commands.  (The same keymap is also the value of `ctl-x-map'.)

   Prefix key definitions can appear in any active keymap.  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.  Major and
minor modes can redefine a key as a prefix by putting a prefix key
definition for it in the local map or the minor mode's map.  Note:
Active Keymaps.

   If a key is defined as a prefix in more than one active map, then its
various definitions are in effect merged: the commands defined in the
minor mode keymaps come first, followed by those in the local map's
prefix definition, and then by those from the global map.

   In the following example, we make `C-p' a prefix key in the local
keymap, in such a way that `C-p' is identical to `C-x'.  Then the
binding for `C-p C-f' is the function `find-file', just like `C-x C-f'.
The key sequence `C-p 6' is not found in any active keymap.

     (use-local-map (make-sparse-keymap))
         => nil
     (local-set-key "\C-p" ctl-x-map)
         => nil
     (key-binding "\C-p\C-f")
         => find-file
     
     (key-binding "\C-p6")
         => nil

 - Function: define-prefix-command symbol &optional mapvar prompt
     This function prepares SYMBOL for use as a prefix key's binding:
     it creates a sparse keymap and stores it as SYMBOL's function
     definition.  Subsequently binding a key sequence to SYMBOL will
     make that key sequence into a prefix key.  The return value is
     `symbol'.

     This function also sets SYMBOL as a variable, with the keymap as
     its value.  But if MAPVAR is non-`nil', it sets MAPVAR as a
     variable instead.

     If PROMPT is non-`nil', that becomes the overall prompt string for
     the keymap.  The prompt string should be given for menu keymaps
     (Note: Defining Menus).