@c This is part of the Emacs manual.
@c Copyright (C) 1985, 1986, 1987, 1993 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@iftex
@chapter Characters, Keys and Commands

  This chapter explains the character set 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 necessary for understanding how
your keyboard input is understood by Emacs.
@end iftex

@node User Input, Keys, Screen, Top
@section Keyboard Input
@cindex input event
@cindex keyboard input
@cindex character set (keyboard)
@cindex ASCII
@cindex C-
@cindex Control

  GNU Emacs uses an extension of the ASCII character set for keyboard
input.  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 and then 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, the shift key is meaningless with control
characters: @kbd{C-a} and @kbd{C-A} are the same character, and Emacs
cannot distinguish them.  Under X Windows, these are distinct
characters, but the standard key bindings treat them the same in all
contexts.

  On ASCII terminals, there are only 32 possible control characters.
These are the control variants of letters and @samp{@@[]\^_}.  Under X
Windows, every non-control character has a control variant.  For
example, @kbd{C-+} and @kbd{C-5} are meaningful under X.

  Emacs extends the ASCII character code by adding an extra bit to each
character.  The additional bit is called Meta.  Any character can be
made Meta; examples of Meta characters 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 traditional reasons,
@kbd{M-C-a} is usually called @kbd{C-M-a}; logically speaking, the order
in which the modifier keys @key{CTRL} and @key{META} are mentioned does
not matter.@refill

@cindex Meta
@cindex M-
@cindex ESC replacing 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
Meta keys, too, in case you have formed a habit of using it.@refill

  X Windows provides several other modifier keys that can be applied to
any keyboard input character.  These are called @key{SUPER}, @key{HYPER}
and @key{ALT}.  With them, you can make characters that we denote with
@samp{s-}, @samp{H-} and @samp{A-}.  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, and the standard key bindings of Emacs do not
include such characters.  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.  These inputs do not have numeric
character codes.  Instead, Emacs represents them by their names
(actually, Lisp objects called @dfn{symbols}).

@cindex input event
  Input characters and non-character inputs are collectively called
@dfn{input events}.  

  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 converts them to names before any other part of Emacs gets to see
them.

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

@cindex key sequence
  A @dfn{key sequence} (@dfn{key}, for short) is a sequence of input
events that combine as part of the invocation of a single command.
Recall that input events include both keyboard characters and
non-character inputs (function keys, arrow keys, mouse buttons, and so
forth).

@cindex complete key
@cindex prefix key
  If the sequence is enough to invoke a command, it is a @dfn{complete
key}.  If it isn't long enough to be complete, we call it a @dfn{prefix
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}.

  Most single characters constitute complete keys in the standard Emacs
command bindings.  A few of them are prefix keys.  A prefix key can be
followed by additional input characters (or other events) to make a
longer key, 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 character 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 any key sequence longer than
one character must be reached through a chain of prefix keys.

  By contrast, 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-x},
@kbd{C-h}, @kbd{C-x C-a}, @kbd{C-x n}, @kbd{C-x r}, @kbd{C-x v},
@kbd{C-x 4}, @kbd{C-x 5}, and @key{ESC}.  But this is not cast in
concrete; it is just a matter of Emacs's standard key bindings.  In
customizing Emacs, you could make new prefix keys, or eliminate these.
@xref{Key Bindings}.@refill

  Whether a sequence is a key can be changed by customization.  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.
  
@node Commands, Text Characters, Keys, Top
@section Keys and Commands

@cindex binding
@cindex customization
@cindex keymap
@cindex function
@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}.  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 customizer the information he needs, 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 accordingly.  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 (usually 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}.

@cindex display table 
  Non-ASCII characters 128 and up are displayed with octal escape
sequences; thus, character code 243 (octal) is displayed as
@samp{\243}.  You can customize the display of these character codes (or
ANSI characters) by creating a @dfn{display table}; this is useful for
editing files that use 8-bit European character sets.  @xref{Display
Tables,, Display Tables, elisp, The Emacs Lisp Reference Manual}.

