1 @c This is part of the Emacs manual.
2 @c Copyright (C) 1985, 1986, 1987, 1993, 1994, 1995, 1997, 2002, 2003,
3 @c 2004, 2005, 2006 Free Software Foundation, Inc.
4 @c See file emacs.texi for copying conditions.
6 @chapter Characters, Keys and Commands
8 This chapter explains the character sets used by Emacs for input
9 commands and for the contents of files, and also explains the concepts
10 of @dfn{keys} and @dfn{commands}, which are fundamental for understanding
11 how Emacs interprets your keyboard and mouse input.
18 @node User Input, Keys, Screen, Top
19 @section Kinds of User Input
20 @cindex input with the keyboard
21 @cindex keyboard input
22 @cindex character set (keyboard)
23 @cindex @acronym{ASCII}
26 @cindex control characters
28 GNU Emacs uses an extension of the @acronym{ASCII} character set for keyboard
29 input; it also accepts non-character input events including function
30 keys and mouse button actions.
32 @acronym{ASCII} consists of 128 character codes. Some of these codes are
33 assigned graphic symbols such as @samp{a} and @samp{=}; the rest are
34 control characters, such as @kbd{Control-a} (usually written @kbd{C-a}
35 for short). @kbd{C-a} gets its name from the fact that you type it by
36 holding down the @key{CTRL} key while pressing @kbd{a}.
38 Some @acronym{ASCII} control characters have special names, and most terminals
39 have special keys you can type them with: for example, @key{RET},
40 @key{TAB}, @key{DEL} and @key{ESC}. The space character is usually
41 referred to below as @key{SPC}, even though strictly speaking it is a
42 graphic character whose graphic happens to be blank.
44 Emacs extends the @acronym{ASCII} character set with thousands more printing
45 characters (@pxref{International}), additional control characters, and a
46 few more modifiers that can be combined with any character.
48 On @acronym{ASCII} terminals, there are only 32 possible control characters.
49 These are the control variants of letters and @samp{@@[]\^_}. In
50 addition, the shift key is meaningless with control characters:
51 @kbd{C-a} and @kbd{C-A} are the same character, and Emacs cannot
54 But the Emacs character set has room for control variants of all
55 printing characters, and for distinguishing between @kbd{C-a} and
56 @kbd{C-A}. Graphical terminals make it possible to enter all these
57 characters. For example, @kbd{C--} (that's Control-Minus) and
58 @kbd{C-5} are meaningful Emacs commands on a graphical terminal.
60 Another Emacs character-set extension is additional modifier bits.
61 Only one modifier bit is commonly used; it is called Meta. Every
62 character has a Meta variant; examples include @kbd{Meta-a} (normally
63 written @kbd{M-a}, for short), @kbd{M-A} (different from @kbd{M-a},
64 but they are normally equivalent in Emacs), @kbd{M-@key{RET}}, and
65 @kbd{M-C-a}. That last means @kbd{a} with both the @key{CTRL} and
66 @key{META} modifiers. We usually write it as @kbd{C-M-a} rather than
67 @kbd{M-C-a}, for reasons of tradition.
71 @cindex @key{ESC} replacing @key{META} key
72 Some terminals have a @key{META} key, and allow you to type Meta
73 characters by holding this key down. Thus, you can type @kbd{Meta-a}
74 by holding down @key{META} and pressing @kbd{a}. The @key{META} key
75 works much like the @key{SHIFT} key. In fact, this key is more often
76 labeled @key{ALT} or @key{EDIT}, instead of @key{META}; on a Sun
77 keyboard, it may have a diamond on it.
79 If there is no @key{META} key, you can still type Meta characters
80 using two-character sequences starting with @key{ESC}. Thus, you can
81 enter @kbd{M-a} by typing @kbd{@key{ESC} a}. You can enter
82 @kbd{C-M-a} by typing @kbd{@key{ESC} C-a}. Unlike @key{META}, which
83 modifies other characters, @key{ESC} is a separate character. You
84 don't hold down @key{ESC} while typing the next character; instead,
85 you press it and release it, then you enter the next character.
86 @key{ESC} is allowed on terminals with @key{META} keys, too, in case
87 you have formed a habit of using it.
89 Emacs defines several other modifier keys that can be applied to any
90 input character. These are called @key{SUPER}, @key{HYPER} and
91 @key{ALT}. We write @samp{s-}, @samp{H-} and @samp{A-} to say that a
92 character uses these modifiers. Thus, @kbd{s-H-C-x} is short for
93 @kbd{Super-Hyper-Control-x}. Not all graphical terminals actually
94 provide keys for these modifier flags---in fact, many terminals have a
95 key labeled @key{ALT} which is really a @key{META} key. The standard
96 key bindings of Emacs do not include any characters with these
97 modifiers. But you can assign them meanings of your own by
100 If your keyboard lacks one of these modifier keys, you can enter it
101 using @kbd{C-x @@}: @kbd{C-x @@ h} adds the ``hyper'' flag to the next
102 character, @kbd{C-x @@ s} adds the ``super'' flag, and @kbd{C-x @@ a}
103 adds the ``alt'' flag. For instance, @kbd{C-x @@ h C-a} is a way to
104 enter @kbd{Hyper-Control-a}. (Unfortunately there is no way to add
105 two modifiers by using @kbd{C-x @@} twice for the same character,
106 because the first one goes to work on the @kbd{C-x}.)
108 Keyboard input includes keyboard keys that are not characters at
109 all: for example function keys and arrow keys. Mouse buttons are also
110 outside the gamut of characters. However, you can modify these events
111 with the modifier keys @key{CTRL}, @key{META}, @key{SUPER},
112 @key{HYPER} and @key{ALT}, just as you can modify keyboard characters.
115 Input characters and non-character inputs are collectively called
116 @dfn{input events}. @xref{Input Events,,, elisp, The Emacs Lisp
117 Reference Manual}, for the full Lisp-level details. If you are not
118 doing Lisp programming, but simply want to redefine the meaning of
119 some characters or non-character events, see @ref{Customization}.
121 @acronym{ASCII} terminals cannot really send anything to the computer except
122 @acronym{ASCII} characters. These terminals use a sequence of characters to
123 represent each function key. But that is invisible to the Emacs user,
124 because the keyboard input routines recognize these special sequences
125 and convert them to function key events before any other part of Emacs
128 @node Keys, Commands, User Input, Top
133 A @dfn{key sequence} (@dfn{key}, for short) is a sequence of input
134 events that are meaningful as a unit---as ``a single command.'' Some
135 Emacs command sequences are just one character or one event; for
136 example, just @kbd{C-f} is enough to move forward one character in the
137 buffer. But Emacs also has commands that take two or more events to
142 If a sequence of events is enough to invoke a command, it is a
143 @dfn{complete key}. Examples of complete keys include @kbd{C-a},
144 @kbd{X}, @key{RET}, @key{NEXT} (a function key), @key{DOWN} (an arrow
145 key), @kbd{C-x C-f}, and @kbd{C-x 4 C-f}. If it isn't long enough to be
146 complete, we call it a @dfn{prefix key}. The above examples show that
147 @kbd{C-x} and @kbd{C-x 4} are prefix keys. Every key sequence is either
148 a complete key or a prefix key.
150 Most single characters constitute complete keys in the standard Emacs
151 command bindings. A few of them are prefix keys. A prefix key combines
152 with the following input event to make a longer key sequence, which may
153 itself be complete or a prefix. For example, @kbd{C-x} is a prefix key,
154 so @kbd{C-x} and the next input event combine to make a two-event
155 key sequence. Most of these key sequences are complete keys, including
156 @kbd{C-x C-f} and @kbd{C-x b}. A few, such as @kbd{C-x 4} and @kbd{C-x
157 r}, are themselves prefix keys that lead to three-event key
158 sequences. There's no limit to the length of a key sequence, but in
159 practice people rarely use sequences longer than four events.
161 By contrast, you can't add more events onto a complete key. For
162 example, the two-event sequence @kbd{C-f C-k} is not a key, because
163 the @kbd{C-f} is a complete key in itself. It's impossible to give
164 @kbd{C-f C-k} an independent meaning as a command. @kbd{C-f C-k} is two
165 key sequences, not one.@refill
167 All told, the prefix keys in Emacs are @kbd{C-c}, @kbd{C-h},
168 @kbd{C-x}, @kbd{C-x @key{RET}}, @kbd{C-x @@}, @kbd{C-x a}, @kbd{C-x
169 n}, @w{@kbd{C-x r}}, @kbd{C-x v}, @kbd{C-x 4}, @kbd{C-x 5}, @kbd{C-x
170 6}, @key{ESC}, @kbd{M-g}, and @kbd{M-o}. (@key{F1} and @key{F2} are
171 aliases for @kbd{C-h} and @kbd{C-x 6}.) But this list is not cast in
172 concrete; it is just a matter of Emacs's standard key bindings. If
173 you customize Emacs, you can make new prefix keys, or eliminate some
174 of the standard ones. @xref{Key Bindings}.
176 If you do make or eliminate prefix keys, that changes the set of
177 possible key sequences. For example, if you redefine @kbd{C-f} as a
178 prefix, @kbd{C-f C-k} automatically becomes a key (complete, unless you
179 define that too as a prefix). Conversely, if you remove the prefix
180 definition of @kbd{C-x 4}, then @kbd{C-x 4 f} (or @kbd{C-x 4
181 @var{anything}}) is no longer a key.
183 Typing the help character (@kbd{C-h} or @key{F1}) after a prefix key
184 displays a list of the commands starting with that prefix. There are
185 a few prefix keys for which @kbd{C-h} does not work---for historical
186 reasons, they define other meanings for @kbd{C-h} which are painful to
187 change. But @key{F1} should work for all prefix keys.
189 @node Commands, Text Characters, Keys, Top
190 @section Keys and Commands
194 @cindex function definition
195 This manual is full of passages that tell you what particular keys
196 do. But Emacs does not assign meanings to keys directly. Instead,
197 Emacs assigns meanings to named @dfn{commands}, and then gives keys
198 their meanings by @dfn{binding} them to commands.
200 Every command has a name chosen by a programmer. The name is usually
201 made of a few English words separated by dashes; for example,
202 @code{next-line} or @code{forward-word}. A command also has a
203 @dfn{function definition} which is a Lisp program; this is what makes
204 the command do what it does. In Emacs Lisp, a command is actually a
205 special kind of Lisp function; one which specifies how to read arguments
206 for it and call it interactively. For more information on commands and
207 functions, see @ref{What Is a Function,, What Is a Function, elisp, The
208 Emacs Lisp Reference Manual}. (The definition we use in this manual is
209 simplified slightly.)
211 The bindings between keys and commands are recorded in various tables
212 called @dfn{keymaps}. @xref{Keymaps}.
214 When we say that ``@kbd{C-n} moves down vertically one line'' we are
215 glossing over a distinction that is irrelevant in ordinary use but is vital
216 in understanding how to customize Emacs. It is the command
217 @code{next-line} that is programmed to move down vertically. @kbd{C-n} has
218 this effect @emph{because} it is bound to that command. If you rebind
219 @kbd{C-n} to the command @code{forward-word} then @kbd{C-n} will move
220 forward by words instead. Rebinding keys is a common method of
221 customization.@refill
223 In the rest of this manual, we usually ignore this distinction to
224 keep things simple. We will often speak of keys like @kbd{C-n} as
225 commands, even though strictly speaking a key is bound to some
226 command. To give the information needed for customization, we state
227 the name of the command which really does the work in parentheses
228 after mentioning the key that runs it. For example, we will say that
229 ``The command @kbd{C-n} (@code{next-line}) moves point vertically
230 down,'' meaning that @code{next-line} is a command that moves
231 vertically down, and @kbd{C-n} is a key that is normally bound to it.
233 While we are on the subject of information for customization only,
234 it's a good time to tell you about @dfn{variables}. Often the
235 description of a command will say, ``To change this, set the variable
236 @code{mumble-foo}.'' A variable is a name used to remember a value.
237 Most of the variables documented in this manual exist just to facilitate
238 customization: some command or other part of Emacs examines the variable
239 and behaves differently according to the value that you set. Until you
240 are interested in customizing, you can ignore the information about
241 variables. When you are ready to be interested, read the basic
242 information on variables, and then the information on individual
243 variables will make sense. @xref{Variables}.
245 @node Text Characters, Entering Emacs, Commands, Top
246 @section Character Set for Text
247 @cindex characters (in text)
249 Text in Emacs buffers is a sequence of 8-bit bytes. Each byte can
250 hold a single @acronym{ASCII} character. Both @acronym{ASCII} control characters (octal
251 codes 000 through 037, and 0177) and @acronym{ASCII} printing characters (codes
252 040 through 0176) are allowed; however, non-@acronym{ASCII} control characters
253 cannot appear in a buffer. The other modifier flags used in keyboard
254 input, such as Meta, are not allowed in buffers either.
256 Some @acronym{ASCII} control characters serve special purposes in text, and have
257 special names. For example, the newline character (octal code 012) is
258 used in the buffer to end a line, and the tab character (octal code 011)
259 is used for indenting to the next tab stop column (normally every 8
260 columns). @xref{Text Display}.
262 Non-@acronym{ASCII} printing characters can also appear in buffers. When
263 multibyte characters are enabled, you can use any of the non-@acronym{ASCII}
264 printing characters that Emacs supports. They have character codes
265 starting at 256, octal 0400, and each one is represented as a sequence
266 of two or more bytes. @xref{International}. Single-byte characters
267 with codes 128 through 255 can also appear in multibyte buffers.
269 If you disable multibyte characters, then you can use only one
270 alphabet of non-@acronym{ASCII} characters, but they all fit in one byte. They
271 use codes 0200 through 0377. @xref{Unibyte Mode}.
278 arch-tag: 9be43eef-d1f4-4d03-a916-c741ea713a45