1 @c This is part of the Emacs manual.
2 @c Copyright (C) 1985, 86, 87, 93, 94, 95, 1997 Free Software Foundation, Inc.
3 @c See file emacs.texi for copying conditions.
5 @chapter Characters, Keys and Commands
7 This chapter explains the character sets used by Emacs for input
8 commands and for the contents of files, and also explains the concepts
9 of @dfn{keys} and @dfn{commands}, which are fundamental for understanding
10 how Emacs interprets your keyboard and mouse input.
13 @node User Input, Keys, Screen, Top
14 @section Kinds of User Input
15 @cindex input with the keyboard
16 @cindex keyboard input
17 @cindex character set (keyboard)
21 @cindex control characters
23 GNU Emacs uses an extension of the ASCII character set for keyboard
24 input; it also accepts non-character input events including function
25 keys and mouse button actions.
27 ASCII consists of 128 character codes. Some of these codes are
28 assigned graphic symbols such as @samp{a} and @samp{=}; the rest are
29 control characters, such as @kbd{Control-a} (usually written @kbd{C-a}
30 for short). @kbd{C-a} gets its name from the fact that you type it by
31 holding down the @key{CTRL} key while pressing @kbd{a}.
33 Some ASCII control characters have special names, and most terminals
34 have special keys you can type them with: for example, @key{RET},
35 @key{TAB}, @key{DEL} and @key{ESC}. The space character is usually
36 referred to below as @key{SPC}, even though strictly speaking it is a
37 graphic character whose graphic happens to be blank. Some keyboards
38 have a key labeled ``linefeed'' which is an alias for @kbd{C-j}.
40 Emacs extends the ASCII character set with thousands more printing
41 characters (@pxref{International}), additional control characters, and a
42 few more modifiers that can be combined with any character.
44 On ASCII terminals, there are only 32 possible control characters.
45 These are the control variants of letters and @samp{@@[]\^_}. In
46 addition, the shift key is meaningless with control characters:
47 @kbd{C-a} and @kbd{C-A} are the same character, and Emacs cannot
50 But the Emacs character set has room for control variants of all
51 printing characters, and for distinguishing between @kbd{C-a} and
52 @kbd{C-A}. The X Window System makes it possible to enter all these
53 characters. For example, @kbd{C--} (that's Control-Minus) and @kbd{C-5}
54 are meaningful Emacs commands under X.
56 Another Emacs character-set extension is additional modifier bits.
57 Only one modifier bit is commonly used; it is called Meta. Every
58 character has a Meta variant; examples include @kbd{Meta-a} (normally
59 written @kbd{M-a}, for short), @kbd{M-A} (not the same character as
60 @kbd{M-a}, but those two characters normally have the same meaning in
61 Emacs), @kbd{M-@key{RET}}, and @kbd{M-C-a}. For reasons of tradition,
62 we usually write @kbd{C-M-a} rather than @kbd{M-C-a}; logically
63 speaking, the order in which the modifier keys @key{CTRL} and @key{META}
64 are mentioned does not matter.
68 @cindex @key{ESC} replacing @key{META} key
69 Some terminals have a @key{META} key, and allow you to type Meta
70 characters by holding this key down. Thus, @kbd{Meta-a} is typed by
71 holding down @key{META} and pressing @kbd{a}. The @key{META} key
72 works much like the @key{SHIFT} key. Such a key is not always labeled
73 @key{META}, however, as this function is often a special option for a
74 key with some other primary purpose. Sometimes it is labeled
75 @key{ALT} or @key{EDIT}; on a Sun keyboard, it may have a diamond on
78 If there is no @key{META} key, you can still type Meta characters
79 using two-character sequences starting with @key{ESC}. Thus, you can
80 enter @kbd{M-a} by typing @kbd{@key{ESC} a}. You can enter
81 @kbd{C-M-a} by typing @kbd{@key{ESC} C-a}. Unlike @key{META}, which
82 modifies other characters, @key{ESC} is a separate character. You
83 don't hold down @key{ESC} while typing the next character; instead,
84 you press it and release it, then you enter the next character.
85 @key{ESC} is allowed on terminals with @key{META} keys, too, in case
86 you have formed a habit of using it.
88 The X Window System provides several other modifier keys that can be
89 applied to any input character. These are called @key{SUPER},
90 @key{HYPER} and @key{ALT}. We write @samp{s-}, @samp{H-} and @samp{A-}
91 to say that a character uses these modifiers. Thus, @kbd{s-H-C-x} is
92 short for @kbd{Super-Hyper-Control-x}. Not all X terminals actually
93 provide keys for these modifier flags---in fact, many terminals have a
94 key labeled @key{ALT} which is really a @key{META} key. The standard
95 key bindings of Emacs do not include any characters with these
96 modifiers. But you can assign them meanings of your own by customizing
99 If your keyboard lacks one of these modifier keys, you can enter it
100 using @kbd{C-x @@}: @kbd{C-x @@ h} adds the ``hyper'' flag to the next
101 character, @kbd{C-x @@ s} adds the ``super'' flag, and @kbd{C-x @@ a}
102 adds the ``alt'' flag. For instance, @kbd{C-x @@ h C-a} is a way to
103 enter @kbd{Hyper-Control-a}. (Unfortunately there is no way to add
104 two modifiers by using @kbd{C-x @@} twice for the same character,
105 because the first one goes to work on the @kbd{C-x}.)
107 Keyboard input includes keyboard keys that are not characters at all:
108 for example function keys and arrow keys. Mouse buttons are also
109 outside the gamut of characters. You can modify these events with the
110 modifier keys @key{CTRL}, @key{META}, @key{SUPER}, @key{HYPER} and
111 @key{ALT}, just like keyboard characters.
114 Input characters and non-character inputs are collectively called
115 @dfn{input events}. @xref{Input Events,,, elisp, The Emacs Lisp
116 Reference Manual}, for more information. If you are not doing Lisp
117 programming, but simply want to redefine the meaning of some characters
118 or non-character events, see @ref{Customization}.
120 ASCII terminals cannot really send anything to the computer except
121 ASCII characters. These terminals use a sequence of characters to
122 represent each function key. But that is invisible to the Emacs user,
123 because the keyboard input routines recognize these special sequences
124 and convert them to function key events before any other part of Emacs
127 @node Keys, Commands, User Input, Top
132 A @dfn{key sequence} (@dfn{key}, for short) is a sequence of input
133 events that are meaningful as a unit---as ``a single command.'' Some
134 Emacs command sequences are just one character or one event; for
135 example, just @kbd{C-f} is enough to move forward one character in the
136 buffer. But Emacs also has commands that take two or more events to
141 If a sequence of events is enough to invoke a command, it is a
142 @dfn{complete key}. Examples of complete keys include @kbd{C-a},
143 @kbd{X}, @key{RET}, @key{NEXT} (a function key), @key{DOWN} (an arrow
144 key), @kbd{C-x C-f}, and @kbd{C-x 4 C-f}. If it isn't long enough to be
145 complete, we call it a @dfn{prefix key}. The above examples show that
146 @kbd{C-x} and @kbd{C-x 4} are prefix keys. Every key sequence is either
147 a complete key or a prefix key.
149 Most single characters constitute complete keys in the standard Emacs
150 command bindings. A few of them are prefix keys. A prefix key combines
151 with the following input event to make a longer key sequence, which may
152 itself be complete or a prefix. For example, @kbd{C-x} is a prefix key,
153 so @kbd{C-x} and the next input event combine to make a two-event
154 key sequence. Most of these key sequences are complete keys, including
155 @kbd{C-x C-f} and @kbd{C-x b}. A few, such as @kbd{C-x 4} and @kbd{C-x
156 r}, are themselves prefix keys that lead to three-event key
157 sequences. There's no limit to the length of a key sequence, but in
158 practice people rarely use sequences longer than four events.
160 By contrast, you can't add more events onto a complete key. For
161 example, the two-event sequence @kbd{C-f C-k} is not a key, because
162 the @kbd{C-f} is a complete key in itself. It's impossible to give
163 @kbd{C-f C-k} an independent meaning as a command. @kbd{C-f C-k} is two
164 key sequences, not one.@refill
166 All told, the prefix keys in Emacs are @kbd{C-c}, @kbd{C-h},
167 @kbd{C-x}, @kbd{C-x @key{RET}}, @kbd{C-x @@}, @kbd{C-x a}, @kbd{C-x n}, @w{@kbd{C-x
168 r}}, @kbd{C-x v}, @kbd{C-x 4}, @kbd{C-x 5}, @kbd{C-x 6}, @key{ESC}, and
169 @kbd{M-g}. But this list is not cast in concrete; it is
170 just a matter of Emacs's standard key bindings. If you customize Emacs,
171 you can make new prefix keys, or eliminate these. @xref{Key Bindings}.
173 If you do make or eliminate prefix keys, that changes the set of
174 possible key sequences. For example, if you redefine @kbd{C-f} as a
175 prefix, @kbd{C-f C-k} automatically becomes a key (complete, unless you
176 define that too as a prefix). Conversely, if you remove the prefix
177 definition of @kbd{C-x 4}, then @kbd{C-x 4 f} (or @kbd{C-x 4
178 @var{anything}}) is no longer a key.
180 Typing the help character (@kbd{C-h} or @key{F1}) after a prefix
181 key displays a list of the commands starting with that prefix.
182 There are a few prefix keys for which @kbd{C-h} does not
183 work---for historical reasons, they have other meanings for @kbd{C-h}
184 which are not easy to change. But @key{F1} should work for all prefix
187 @node Commands, Text Characters, Keys, Top
188 @section Keys and Commands
192 @cindex function definition
193 This manual is full of passages that tell you what particular keys
194 do. But Emacs does not assign meanings to keys directly. Instead,
195 Emacs assigns meanings to named @dfn{commands}, and then gives keys
196 their meanings by @dfn{binding} them to commands.
198 Every command has a name chosen by a programmer. The name is usually
199 made of a few English words separated by dashes; for example,
200 @code{next-line} or @code{forward-word}. A command also has a
201 @dfn{function definition} which is a Lisp program; this is what makes
202 the command do what it does. In Emacs Lisp, a command is actually a
203 special kind of Lisp function; one which specifies how to read arguments
204 for it and call it interactively. For more information on commands and
205 functions, see @ref{What Is a Function,, What Is a Function, elisp, The
206 Emacs Lisp Reference Manual}. (The definition we use in this manual is
207 simplified slightly.)
209 The bindings between keys and commands are recorded in various tables
210 called @dfn{keymaps}. @xref{Keymaps}.
212 When we say that ``@kbd{C-n} moves down vertically one line'' we are
213 glossing over a distinction that is irrelevant in ordinary use but is vital
214 in understanding how to customize Emacs. It is the command
215 @code{next-line} that is programmed to move down vertically. @kbd{C-n} has
216 this effect @emph{because} it is bound to that command. If you rebind
217 @kbd{C-n} to the command @code{forward-word} then @kbd{C-n} will move
218 forward by words instead. Rebinding keys is a common method of
219 customization.@refill
221 In the rest of this manual, we usually ignore this subtlety to keep
222 things simple. To give the information needed for customization, we
223 state the name of the command which really does the work in parentheses
224 after mentioning the key that runs it. For example, we will say that
225 ``The command @kbd{C-n} (@code{next-line}) moves point vertically
226 down,'' meaning that @code{next-line} is a command that moves vertically
227 down, and @kbd{C-n} is a key that is normally bound to it.
229 While we are on the subject of information for customization only,
230 it's a good time to tell you about @dfn{variables}. Often the
231 description of a command will say, ``To change this, set the variable
232 @code{mumble-foo}.'' A variable is a name used to remember a value.
233 Most of the variables documented in this manual exist just to facilitate
234 customization: some command or other part of Emacs examines the variable
235 and behaves differently according to the value that you set. Until you
236 are interested in customizing, you can ignore the information about
237 variables. When you are ready to be interested, read the basic
238 information on variables, and then the information on individual
239 variables will make sense. @xref{Variables}.
241 @node Text Characters, Entering Emacs, Commands, Top
242 @section Character Set for Text
243 @cindex characters (in text)
245 Text in Emacs buffers is a sequence of 8-bit bytes. Each byte can
246 hold a single ASCII character. Both ASCII control characters (octal
247 codes 000 through 037, and 0177) and ASCII printing characters (codes
248 040 through 0176) are allowed; however, non-ASCII control characters
249 cannot appear in a buffer. The other modifier flags used in keyboard
250 input, such as Meta, are not allowed in buffers either.
252 Some ASCII control characters serve special purposes in text, and have
253 special names. For example, the newline character (octal code 012) is
254 used in the buffer to end a line, and the tab character (octal code 011)
255 is used for indenting to the next tab stop column (normally every 8
256 columns). @xref{Text Display}.
258 Non-ASCII printing characters can also appear in buffers. When
259 multibyte characters are enabled, you can use any of the non-ASCII
260 printing characters that Emacs supports. They have character codes
261 starting at 256, octal 0400, and each one is represented as a sequence
262 of two or more bytes. @xref{International}. Single-byte characters
263 with codes 128 through 255 can also appear in multibyte buffers.
265 If you disable multibyte characters, then you can use only one
266 alphabet of non-ASCII characters, but they all fit in one byte. They
267 use codes 0200 through 0377. @xref{Single-Byte Character Support}.