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:
display-bindings), since the main use of this prefix key is for mode-specific bindings.
2C-mode-mapis the global keymap used for the C-x 6 prefix key.
vc-prefix-mapis the global keymap used for the C-x v prefix key.
facemenu-keymapis the global keymap used for the M-o prefix key.
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
Control-X-prefix, whose function cell holds the keymap
for C-x commands. (The same keymap is also the value of
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. See 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
like C-x C-f. The key sequence C-p 6 is not found in any
(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
define-prefix-commandsymbol &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
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
(see Defining Menus).