@c This is part of the Emacs manual.
@c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995 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

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

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

  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 control characters have special names, and special keys you can
type them with: for example, @key{RET}, @key{TAB}, @key{LFD}, @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.@refill

  On 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
characters, and for distinguishing between @kbd{C-a} and @kbd{C-A}.  X
Windows makes it possible to enter all these characters.  For example,
@kbd{C--} (that's Control-Minus) and @kbd{C-5} are meaningful Emacs
commands under X.

  Another Emacs character set extension is that characters have
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} (not the
same character as @kbd{M-a}, but those two characters normally have the
same meaning in Emacs), @kbd{M-@key{RET}}, and @kbd{M-C-a}.  For reasons
of tradition, we usually write @kbd{C-M-a} rather than @kbd{M-C-a};
logically speaking, the order in which the modifier keys @key{CTRL} and
@key{META} are mentioned does not matter.

@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, @kbd{Meta-a} is typed by
holding down @key{META} and pressing @kbd{a}.  The @key{META} key works
much like the @key{SHIFT} key.  Such a key is not always labeled
@key{META}, however, as this function is often a special option for a key
with some other primary purpose.@refill

  If there is no @key{META} key, you can still type Meta characters
using two-character sequences starting with @key{ESC}.  Thus, to enter
@kbd{M-a}, you could type @kbd{@key{ESC} a}.  To enter @kbd{C-M-a}, you
would type @kbd{@key{ESC} C-a}.  @key{ESC} is allowed on terminals with
@key{META} keys, too, in case you have formed a habit of using it.
  
  X Windows provides 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 X 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.

  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.  You can modify these events with the
modifier keys @key{CONTROL}, @key{META}, @key{SUPER}, @key{HYPER} and
@key{ALT} like keyboard characters.

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

  ASCII terminals cannot really send anything to the computer except
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.  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-character
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-character 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-character 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 C-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}, and @key{ESC}.  But this 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
these.  @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 it 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
character displays a list of the commands starting with that prefix.
There are a few prefix characters for which @kbd{C-h} does not
work---for historical reasons, they have other meanings for @kbd{C-h}
which are not easy to change.  But @key{F1} should work for all prefix
characters.
  
@node Commands, Text Characters, Keys, Top
@section Keys and Commands

@cindex binding
@cindex function
@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 subtlety to keep
things simple.  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 standardly 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)

  Emacs buffers use an 8-bit character set, because bytes have 8 bits.
ASCII graphic characters in Emacs buffers are displayed with their
graphics.  The newline character (which has the same character code as
@key{LFD}) is displayed by starting a new line.  The tab character is
displayed by moving to the next tab stop column (normally every 8
columns).  Other control characters are displayed as a caret (@samp{^})
followed by the non-control version of the character; thus, @kbd{C-a} is
displayed as @samp{^A}.  Non-ASCII characters 128 and up are displayed
with octal escape sequences; thus, character code 243 (octal) is
displayed as @samp{\243}.

@cindex display table
  You can customize the display of these character codes (or
ASCII characters) by creating a @dfn{display table}.  @xref{Display
Tables,, Display Tables, elisp, The Emacs Lisp Reference Manual}.
This is useful for
editing files that use 8-bit European character sets.
@xref{European Display}.
