@c This is part of the Emacs manual.
@c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1997, 2002, 2003,
@c   2004, 2005, 2006 Free Software 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 for the contents of files, and also explains the concepts
of @dfn{keys} and @dfn{commands}, which are fundamental for understanding
how Emacs interprets your keyboard and mouse input.
@end iftex

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@node User Input, Keys, Screen, Top
@section Kinds of User Input
@cindex input with the keyboard
@cindex keyboard input
@cindex character set (keyboard)
@cindex @acronym{ASCII}
@cindex C-
@cindex Control
@cindex control characters

  GNU Emacs uses an extension of the @acronym{ASCII} character set for keyboard
input; it also accepts non-character input events including function
keys and mouse button actions.

  @acronym{ASCII} consists of 128 character codes.  Some of these codes are
assigned graphic symbols such as @samp{a} and @samp{=}; the rest are
control characters, such as @kbd{Control-a} (usually written @kbd{C-a}
for short).  @kbd{C-a} gets its name from the fact that you type it by
holding down the @key{CTRL} key while pressing @kbd{a}.

  Some @acronym{ASCII} control characters have special names, and most terminals
have special keys you can type them with: for example, @key{RET},
@key{TAB}, @key{DEL} and @key{ESC}.  The space character is usually
referred to below as @key{SPC}, even though strictly speaking it is a
graphic character whose graphic happens to be blank.

  Emacs extends the @acronym{ASCII} character set with thousands more printing
characters (@pxref{International}), additional control characters, and a
few more modifiers that can be combined with any character.

  On @acronym{ASCII} terminals, there are only 32 possible control characters.
These are the control variants of letters and @samp{@@[]\^_}.  In
addition, the shift key is meaningless with control characters:
@kbd{C-a} and @kbd{C-A} are the same character, and Emacs cannot
distinguish them.

  But the Emacs character set has room for control variants of all
printing characters, and for distinguishing between @kbd{C-a} and
@kbd{C-A}.  Graphical terminals make it possible to enter all these
characters.  For example, @kbd{C--} (that's Control-Minus) and
@kbd{C-5} are meaningful Emacs commands on a graphical terminal.

  Another Emacs character-set extension is additional modifier bits.
Only one modifier bit is commonly used; it is called Meta.  Every
character has a Meta variant; examples include @kbd{Meta-a} (normally
written @kbd{M-a}, for short), @kbd{M-A} (different from @kbd{M-a},
but they are normally equivalent in Emacs), @kbd{M-@key{RET}}, and
@kbd{M-C-a}.  That last means @kbd{a} with both the @key{CTRL} and
@key{META} modifiers.  We usually write it as @kbd{C-M-a} rather than
@kbd{M-C-a}, for reasons of tradition.

@cindex Meta
@cindex M-
@cindex @key{ESC} replacing @key{META} key
  Some terminals have a @key{META} key, and allow you to type Meta
characters by holding this key down.  Thus, you can type @kbd{Meta-a}
by holding down @key{META} and pressing @kbd{a}.  The @key{META} key
works much like the @key{SHIFT} key.  In fact, this key is more often
labeled @key{ALT} or @key{EDIT}, instead of @key{META}; on a Sun
keyboard, it may have a diamond on it.

  If there is no @key{META} key, you can still 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} by typing @kbd{@key{ESC} C-a}.  Unlike @key{META}, which
modifies other characters, @key{ESC} is a separate character.  You
don't hold down @key{ESC} while typing the next character; instead,
you press it and release it, then you enter the next character.
@key{ESC} is allowed on terminals with @key{META} keys, too, in case
you have formed a habit of using it.

  Emacs defines several other modifier keys that can be applied to any
input character.  These are called @key{SUPER}, @key{HYPER} and
@key{ALT}.  We write @samp{s-}, @samp{H-} and @samp{A-} to say that a
character uses these modifiers.  Thus, @kbd{s-H-C-x} is short for
@kbd{Super-Hyper-Control-x}.  Not all graphical terminals actually
provide keys for these modifier flags---in fact, many terminals have a
key labeled @key{ALT} which is really a @key{META} key.  The standard
key bindings of Emacs do not include any characters with these
modifiers.  But you can assign them meanings of your own by
customizing Emacs.

  If your keyboard lacks one of these modifier keys, you can enter it
using @kbd{C-x @@}: @kbd{C-x @@ h} adds the ``hyper'' flag to the next
character, @kbd{C-x @@ s} adds the ``super'' flag, and @kbd{C-x @@ a}
adds the ``alt'' flag.  For instance, @kbd{C-x @@ h C-a} is a way to
enter @kbd{Hyper-Control-a}.  (Unfortunately there is no way to add
two modifiers by using @kbd{C-x @@} twice for the same character,
because the first one goes to work on the @kbd{C-x}.)

  Keyboard input includes keyboard keys that are not characters at
all: for example function keys and arrow keys.  Mouse buttons are also
outside the gamut of characters.  However, you can modify these events
with the modifier keys @key{CTRL}, @key{META}, @key{SUPER},
@key{HYPER} and @key{ALT}, just as you can modify keyboard characters.

@cindex input event
  Input characters and non-character inputs are collectively called
@dfn{input events}.  @xref{Input Events,,, elisp, The Emacs Lisp
Reference Manual}, for the full Lisp-level details.  If you are not
doing Lisp programming, but simply want to redefine the meaning of
some characters or non-character events, see @ref{Customization}.

  @acronym{ASCII} terminals cannot really send anything to the computer except
@acronym{ASCII} characters.  These terminals use a sequence of characters to
represent each function key.  But that is invisible to the Emacs user,
because the keyboard input routines recognize these special sequences
and convert them to function key events before any other part of Emacs
gets to see them.

@node Keys, Commands, User Input, Top
@section Keys

@cindex key sequence
@cindex key
  A @dfn{key sequence} (@dfn{key}, for short) is a sequence of input
events that are meaningful as a unit---as ``a single command.''  Some
Emacs command sequences are just one character or one event; for
example, just @kbd{C-f} is enough to move forward one character in the
buffer.  But Emacs also has commands that take two or more events to
invoke.

@cindex complete key
@cindex prefix key
  If a sequence of events is enough to invoke a command, it is a
@dfn{complete key}.  Examples of complete keys include @kbd{C-a},
@kbd{X}, @key{RET}, @key{NEXT} (a function key), @key{DOWN} (an arrow
key), @kbd{C-x C-f}, and @kbd{C-x 4 C-f}.  If it isn't long enough to be
complete, we call it a @dfn{prefix key}.  The above examples show that
@kbd{C-x} and @kbd{C-x 4} are prefix keys.  Every key sequence is either
a complete key or a prefix key.

  Most single characters constitute complete keys in the standard Emacs
command bindings.  A few of them are prefix keys.  A prefix key combines
with the following input event to make a longer key sequence, which may
itself be complete or a prefix.  For example, @kbd{C-x} is a prefix key,
so @kbd{C-x} and the next input event combine to make a two-event
key sequence.  Most of these key sequences are complete keys, including
@kbd{C-x C-f} and @kbd{C-x b}.  A few, such as @kbd{C-x 4} and @kbd{C-x
r}, are themselves prefix keys that lead to three-event key
sequences.  There's no limit to the length of a key sequence, but in
practice people rarely use sequences longer than four events.

  By contrast, you can't add more events onto a complete key.  For
example, the two-event sequence @kbd{C-f C-k} is not a key, because
the @kbd{C-f} is a complete key in itself.  It's impossible to give
@kbd{C-f C-k} an independent meaning as a command.  @kbd{C-f C-k} is two
key sequences, not one.@refill

  All told, 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}, @w{@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}.)  But this list is not cast in
concrete; it is just a matter of Emacs's standard key bindings.  If
you customize Emacs, you can make new prefix keys, or eliminate some
of the standard ones.  @xref{Key Bindings}.

  If you do make or eliminate prefix keys, that changes the set of
possible key sequences.  For example, if you redefine @kbd{C-f} as a
prefix, @kbd{C-f C-k} automatically becomes a key (complete, unless you
define that too as a prefix).  Conversely, if you remove the prefix
definition of @kbd{C-x 4}, then @kbd{C-x 4 f} (or @kbd{C-x 4
@var{anything}}) is no longer a key.

  Typing the help character (@kbd{C-h} or @key{F1}) after a prefix key
displays a list of the commands starting with that prefix.  There are
a few prefix keys for which @kbd{C-h} does not work---for historical
reasons, they define other meanings for @kbd{C-h} which are painful to
change.  But @key{F1} should work for all prefix keys.

@node Commands, Text Characters, Keys, Top
@section Keys and Commands

@cindex binding
@cindex command
@cindex function definition
  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}.  A command also has a
@dfn{function definition} which is a Lisp program; this is what makes
the command do what it does.  In Emacs Lisp, a command is actually a
special kind of Lisp function; one which specifies how to read arguments
for it and call it interactively.  For more information on commands and
functions, see @ref{What Is a Function,, What Is a Function, elisp, The
Emacs Lisp Reference Manual}.  (The definition we use in this manual is
simplified slightly.)

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

  When we say that ``@kbd{C-n} moves down vertically one line'' we are
glossing over a distinction that is irrelevant in ordinary use but is vital
in understanding how to customize Emacs.  It is the command
@code{next-line} that is programmed to move down vertically.  @kbd{C-n} has
this effect @emph{because} it is bound to that command.  If you rebind
@kbd{C-n} to the command @code{forward-word} then @kbd{C-n} will move
forward by words instead.  Rebinding keys is a common method of
customization.@refill

  In the rest of this manual, we usually ignore this distinction to
keep things simple.  We will often speak of keys like @kbd{C-n} as
commands, even though strictly speaking a key is bound to some
command.  To give the information needed for customization, 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 @code{next-line} is a command that moves
vertically down, and @kbd{C-n} is a key that is normally bound to it.

  While we are on the subject of information for customization only,
it's a good time to 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 remember a value.
Most of the variables documented in this manual exist just to facilitate
customization: some command or other part of Emacs examines the variable
and behaves differently according to the value that you set.  Until you
are interested in customizing, you can ignore the information about
variables.  When you are ready to be interested, read the basic
information on variables, and then the information on individual
variables will make sense.  @xref{Variables}.

@node Text Characters, Entering Emacs, Commands, Top
@section Character Set for Text
@cindex characters (in text)

  Text in Emacs buffers is a sequence of 8-bit bytes.  Each byte can
hold a single @acronym{ASCII} character.  Both @acronym{ASCII} control characters (octal
codes 000 through 037, and 0177) and @acronym{ASCII} printing characters (codes
040 through 0176) are allowed; however, non-@acronym{ASCII} control characters
cannot appear in a buffer.  The other modifier flags used in keyboard
input, such as Meta, are not allowed in buffers either.

  Some @acronym{ASCII} control characters serve special purposes in text, and have
special names.  For example, the newline character (octal code 012) is
used in the buffer to end a line, and the tab character (octal code 011)
is used for indenting to the next tab stop column (normally every 8
columns).  @xref{Text Display}.

  Non-@acronym{ASCII} printing characters can also appear in buffers.  When
multibyte characters are enabled, you can use any of the non-@acronym{ASCII}
printing characters that Emacs supports.  They have character codes
starting at 256, octal 0400, and each one is represented as a sequence
of two or more bytes.  @xref{International}.  Single-byte characters
with codes 128 through 255 can also appear in multibyte buffers.

  If you disable multibyte characters, then you can use only one
alphabet of non-@acronym{ASCII} characters, but they all fit in one byte.  They
use codes 0200 through 0377.  @xref{Unibyte Mode}.

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