@c This is part of the Emacs manual.
@c Copyright (C) 1985-1987, 1993-1995, 1997, 2001-2018 Free Software
@c Foundation, Inc.
@c See file emacs.texi for copying conditions.
@iftex
@chapter Characters, Keys and Commands

  This chapter explains the character sets used by Emacs for input
commands, and the fundamental concepts of @dfn{keys} and
@dfn{commands}, whereby Emacs interprets your keyboard and mouse
input.
@end iftex

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@node User Input
@section Kinds of User Input
@cindex input with the keyboard
@cindex keyboard input
@cindex character set (keyboard)
@cindex @acronym{ASCII}

  GNU Emacs is primarily designed for use with the keyboard.  While it
is possible to use the mouse to issue editing commands through the
menu bar and tool bar, that is not as efficient as using the keyboard.
Therefore, this manual mainly documents how to edit with the keyboard.

@cindex control character
  Keyboard input into Emacs is based on a heavily-extended version of
@acronym{ASCII}.  Simple characters, like @samp{a}, @samp{B},
@samp{3}, @samp{=}, and the space character (denoted as @key{SPC}),
are entered by typing the corresponding key.  @dfn{Control
characters}, such as @key{RET}, @key{TAB}, @key{DEL}, @key{ESC},
@key{F1}, @key{Home}, and @key{LEFT}, are also entered this way, as
are certain characters found on non-English keyboards
(@pxref{International}).

@cindex modifier keys
@cindex Control
@cindex C-
@cindex META
@cindex M-
  Emacs also recognizes control characters that are entered using
@dfn{modifier keys}.  Two commonly-used modifier keys are
@key{Control} (usually labeled @key{Ctrl}), and @key{Meta} (usually
labeled @key{Alt})@footnote{We refer to @key{Alt} as @key{Meta} for
historical reasons.}.  For example, @kbd{Control-a} is entered by
holding down the @key{Ctrl} key while pressing @kbd{a}; we will refer
to this as @kbd{C-a} for short.  Similarly, @kbd{@key{Meta}-a}, or @kbd{M-a}
for short, is entered by holding down the @key{Alt} key and pressing
@kbd{a}.  Modifier keys can also be applied to non-alphanumerical
characters, e.g., @kbd{C-@key{F1}} or @kbd{M-@key{LEFT}}.

@cindex @key{ESC} replacing @key{Meta} key
  You can also type Meta characters using two-character sequences
starting with @key{ESC}.  Thus, you can enter @kbd{M-a} by typing
@kbd{@key{ESC} a}.  You can enter @kbd{C-M-a} (holding down both
@key{Ctrl} and @key{Alt}, then pressing @kbd{a}) by typing
@kbd{@key{ESC} C-a}.  Unlike @key{Meta}, @key{ESC} is entered as a
separate character.  You don't hold down @key{ESC} while typing the
next character; instead, press @key{ESC} and release it, then enter
the next character.  This feature is useful on certain text terminals
where the @key{Meta} key does not function reliably.

@cindex keys stolen by window manager
@cindex window manager, keys stolen by
  On graphical displays, the window manager might block some keyboard
inputs, including @kbd{M-@key{TAB}}, @kbd{M-@key{SPC}}, @kbd{C-M-d}
and @kbd{C-M-l}.  If you have this problem, you can either customize
your window manager to not block those keys, or rebind the
affected Emacs commands (@pxref{Customization}).

@cindex input event
  Simple characters and control characters, as well as certain
non-keyboard inputs such as mouse clicks, are collectively referred to
as @dfn{input events}.  For details about how Emacs internally handles
input events, see @ref{Input Events,,, elisp, The Emacs Lisp Reference
Manual}.

@node Keys
@section Keys

  Some Emacs commands are invoked by just one input event; for
example, @kbd{C-f} moves forward one character in the buffer.  Other
commands take two or more input events to invoke, such as @kbd{C-x
C-f} and @kbd{C-x 4 C-f}.

@cindex key
@cindex key sequence
@cindex complete key
@cindex prefix key
  A @dfn{key sequence}, or @dfn{key} for short, is a sequence of one
or more input events that is meaningful as a unit.  If a key sequence
invokes a command, we call it a @dfn{complete key}; for example,
@kbd{C-f}, @kbd{C-x C-f} and @kbd{C-x 4 C-f} are all complete keys.
If a key sequence isn't long enough to invoke a command, we call it a
@dfn{prefix key}; from the preceding example, we see that @kbd{C-x}
and @kbd{C-x 4} are prefix keys.  Every key sequence is either a
complete key or a prefix key.

  A prefix key combines with the following input event to make a
longer key sequence.  For example, @kbd{C-x} is a prefix key, so
typing @kbd{C-x} alone does not invoke a command; instead, Emacs waits
for further input (if you pause for longer than a second, it echoes
the @kbd{C-x} key to prompt for that input; @pxref{Echo Area}).
@kbd{C-x} combines with the next input event to make a two-event key
sequence, which could itself be a prefix key (such as @kbd{C-x 4}), or
a complete key (such as @kbd{C-x C-f}).  There is no limit to the
length of key sequences, but in practice they are seldom longer than
three or four input events.

  You can't add input events onto a complete key.  For example,
because @kbd{C-f} is a complete key, the two-event sequence @kbd{C-f
C-k} is two key sequences, not one.

  By default, the prefix keys in Emacs are @kbd{C-c}, @kbd{C-h},
@kbd{C-x}, @kbd{C-x @key{RET}}, @kbd{C-x @@}, @kbd{C-x a}, @kbd{C-x
n}, @kbd{C-x r}, @kbd{C-x v}, @kbd{C-x 4}, @kbd{C-x 5}, @kbd{C-x 6},
@key{ESC}, @kbd{M-g}, and @kbd{M-o}.  (@key{F1} and @key{F2} are
aliases for @kbd{C-h} and @kbd{C-x 6}.)  This list is not cast in
stone; if you customize Emacs, you can make new prefix keys.  You
could even eliminate some of the standard ones, though this is not
recommended for most users; for example, if you remove the prefix
definition of @kbd{C-x 4}, then @kbd{C-x 4 C-f} becomes an invalid key
sequence.  @xref{Key Bindings}.

  Typing the help character (@kbd{C-h} or @key{F1}) after a prefix key
displays a list of the commands starting with that prefix.  The sole
exception to this rule is @key{ESC}: @kbd{@key{ESC} C-h} is equivalent
to @kbd{C-M-h}, which does something else entirely.  You can, however,
use @key{F1} to display a list of commands starting with @key{ESC}.

@node Commands
@section Keys and Commands

@cindex binding
@cindex command
  This manual is full of passages that tell you what particular keys
do.  But Emacs does not assign meanings to keys directly.  Instead,
Emacs assigns meanings to named @dfn{commands}, and then gives keys
their meanings by @dfn{binding} them to commands.

  Every command has a name chosen by a programmer.  The name is
usually made of a few English words separated by dashes; for example,
@code{next-line} or @code{forward-word}.  Internally, each command is
a special type of Lisp @dfn{function}, and the actions associated with
the command are performed by running the function.  @xref{What Is a
Function,, What Is a Function, elisp, The Emacs Lisp Reference
Manual}.

  The bindings between keys and commands are recorded in tables called
@dfn{keymaps}.  @xref{Keymaps}.

  When we say that ``@kbd{C-n} moves down vertically one line'' we are
glossing over a subtle distinction that is irrelevant in ordinary use,
but vital for Emacs customization.  The command @code{next-line} does
a vertical move downward.  @kbd{C-n} has this effect @emph{because} it
is bound to @code{next-line}.  If you rebind @kbd{C-n} to the command
@code{forward-word}, @kbd{C-n} will move forward one word instead.

  In this manual, we will often speak of keys like @kbd{C-n} as
commands, even though strictly speaking the key is bound to a command.
Usually, we state the name of the command which really does the work
in parentheses after mentioning the key that runs it.  For example, we
will say that ``The command @kbd{C-n} (@code{next-line}) moves point
vertically down'', meaning that the command @code{next-line} moves
vertically down, and the key @kbd{C-n} is normally bound to it.

  Since we are discussing customization, we should tell you about
@dfn{variables}.  Often the description of a command will say, ``To
change this, set the variable @code{mumble-foo}.''  A variable is a
name used to store a value.  Most of the variables documented in this
manual are meant for customization: some command or other part of
Emacs examines the variable and behaves differently according to the
value that you set.  You can ignore the information about variables
until you are interested in customizing them.  Then read the basic
information on variables (@pxref{Variables}) and the information about
specific variables will make sense.

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