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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
distinguish them.
But the Emacs character set has room for control variants of all printing characters, and for distinguishing between C-a and C-A. X Windows 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.
If there is no META key, you can still type Meta characters
using two-character sequences starting with ESC. Thus, to enter
M-a, you could type ESC a. To enter C-M-a, you
would type ESC C-a. ESC is allowed on terminals with
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 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 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 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.
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