2 Kinds of User Input
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 ‘a’ and ‘=’; the rest are
control characters, such as Control-a (usually written C-a
for short). C-a gets its name from the fact that you type it by
holding down the <CTRL> key while pressing a.
Some ASCII control characters have special names, and most terminals
have special keys you can type them with: for example, <RET>,
<TAB>, <DEL> and <ESC>. The space character is usually
referred to below as <SPC>, even though strictly speaking it is a
graphic character whose graphic happens to be blank. Some keyboards
have a key labeled “linefeed” which is an alias for C-j.
Emacs extends the ASCII character set with thousands more printing
characters (see International), additional control characters, and a
few more modifiers that can be combined with any character.
On ASCII terminals, there are only 32 possible control characters.
These are the control variants of letters and ‘@\^_’. In
addition, the shift key is meaningless with control characters:
C-a and C-A are the same character, and Emacs cannot
But the Emacs character set has room for control variants of all
printing characters, and for distinguishing between C-a and
C-A. The X Window System makes it possible to enter all these
characters. For example, C-- (that's Control-Minus) and C-5
are meaningful Emacs commands under X.
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 Meta-a (normally
written M-a, for short), M-A (not the same character as
M-a, but those two characters normally have the same meaning in
Emacs), M-<RET>, and M-C-a. For reasons of tradition,
we usually write C-M-a rather than M-C-a; logically
speaking, the order in which the modifier keys <CTRL> and <META>
are mentioned does not matter.
Some terminals have a <META> key, and allow you to type Meta
characters by holding this key down. Thus, Meta-a is typed by
holding down <META> and pressing a. The <META> key
works much like the <SHIFT> key. Such a key is not always labeled
<META>, however, as this function is often a special option for a
key with some other primary purpose. Sometimes it is labeled
<ALT> or <EDIT>; on a Sun keyboard, it may have a diamond on
If there is no <META> key, you can still type Meta characters
using two-character sequences starting with <ESC>. Thus, you can
enter M-a by typing <ESC> a. You can enter
C-M-a by typing <ESC> C-a. Unlike <META>, which
modifies other characters, <ESC> is a separate character. You
don't hold down <ESC> while typing the next character; instead,
you press it and release it, then you enter the next character.
<ESC> is allowed on terminals with <META> keys, too, in case
you have formed a habit of using it.
The X Window System provides several other modifier keys that can be
applied to any input character. These are called <SUPER>,
<HYPER> and <ALT>. We write ‘s-’, ‘H-’ and ‘A-’
to say that a character uses these modifiers. Thus, s-H-C-x is
short for Super-Hyper-Control-x. Not all X terminals actually
provide keys for these modifier flags—in fact, many terminals have a
key labeled <ALT> which is really a <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
If your keyboard lacks one of these modifier keys, you can enter it
using C-x @: C-x @ h adds the “hyper” flag to the next
character, C-x @ s adds the “super” flag, and C-x @ a
adds the “alt” flag. For instance, C-x @ h C-a is a way to
enter Hyper-Control-a. (Unfortunately there is no way to add
two modifiers by using C-x @ twice for the same character,
because the first one goes to work on the 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. You can modify these events with the
modifier keys <CTRL>, <META>, <SUPER>, <HYPER> and
<ALT>, just like keyboard characters.
Input characters and non-character inputs are collectively called
input events. See Input Events, 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 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.