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