57.4.2 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
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
Control-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 Menu Bar, for special information about menu bar key bindings.
Mouse button events that invoke pop-up menus are also prefix keys; see
Menu Keymaps, for more
Some prefix keymaps are stored in variables with names:
ctl-x-map is the variable name for the map used for characters that
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.