Kinds of User Input
===================

GNU Emacs is primarily designed for use with the keyboard.  While it is possible to use the mouse to issue editing commands through the menu bar and tool bar, that is not as efficient as using the keyboard. Therefore, this manual mainly documents how to edit with the keyboard.

Keyboard input into Emacs is based on a heavily-extended version of ASCII.  Simple characters, like a, B, 3, =, and the space character (denoted as SPC), are entered by typing the corresponding key.  Control characters, such as RET, TAB, DEL, ESC, F1, Home, and left, are also entered this way, as are certain characters found on non-English keyboards (see International).

Emacs also recognizes control characters that are entered using modifier keys.  Two commonly-used modifier keys are Control (usually labeled Ctrl), and Meta (usually labeled Alt) (We refer to Alt as Meta for historical reasons.).  For example, Control-a is entered by holding down the Ctrl key while pressing a; we will refer to this as C-a for short.  Similarly Meta-a, or M-a for short, is entered by holding down the Alt key and pressing a.  Modifier keys can also be applied to non-alphanumerical characters, e.g., C-F1 or M-left.

You can also 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, ESC is entered as a separate character.  You don't hold down ESC while typing the next character; instead, press ESC and release it, then enter the next character.  This feature is useful on certain text terminals where the Meta key does not function reliably.

On graphical displays, the window manager might block some keyboard inputs, including M-TAB, M-SPC, C-M-d and C-M-l.  If you have this problem, you can either customize your window manager to not block those keys, or ``rebind'' the affected Emacs commands (see Customization).

Simple characters and control characters, as well as certain non-keyboard inputs such as mouse clicks, are collectively referred to as input events.  For details about how Emacs internally handles input events, see Input Events (The Emacs Lisp Reference Manual).

Keys
====

Some Emacs commands are invoked by just one input event; for example, C-f moves forward one character in the buffer.  Other commands take two or more input events to invoke, such as C-x C-f and C-x 4 C-f.

A key sequence, or key for short, is a sequence of one or more input events that is meaningful as a unit.  If a key sequence invokes a command, we call it a complete key; for example, C-f, C-x C-f and C-x 4 C-f are all complete keys. If a key sequence isn't long enough to invoke a command, we call it a prefix key; from the preceding example, we see that C-x and C-x 4 are prefix keys.  Every key sequence is either a complete key or a prefix key.

A prefix key combines with the following input event to make a longer key sequence.  For example, C-x is a prefix key, so typing C-x alone does not invoke a command; instead, Emacs waits for further input (if you pause for longer than a second, it echoes the C-x key to prompt for that input; see Echo Area). C-x combines with the next input event to make a two-event key sequence, which could itself be a prefix key (such as C-x 4), or a complete key (such as C-x C-f).  There is no limit to the length of key sequences, but in practice they are seldom longer than three or four input events.

You can't add input events onto a complete key.  For example, because C-f is a complete key, the two-event sequence C-f C-k is two key sequences, not one.

By default, the prefix keys in Emacs are C-c, C-h, C-x, C-x RET, C-x @, C-x a, C-x n, C-x r, C-x v, C-x 4, C-x 5, C-x 6, ESC, M-g, and M-o.  (F1 and F2 are aliases for C-h and C-x 6.)  This list is not cast in stone; if you customize Emacs, you can make new prefix keys.  You could even eliminate some of the standard ones, though this is not recommended for most users; for example, if you remove the prefix definition of C-x 4, then C-x 4 C-f becomes an invalid key sequence.  See Key Bindings.

Typing the help character (C-h or F1) after a prefix key displays a list of the commands starting with that prefix.  The sole exception to this rule is ESC: ESC C-h is equivalent to C-M-h, which does something else entirely.  You can, however, use F1 to display a list of commands starting with ESC.

Keys and Commands
=================

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 commands, and then gives keys their meanings by 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, next-line or forward-word.  Internally, each command is a special type of Lisp function, and the actions associated with the command are performed by running the function.  See What Is a Function (The Emacs Lisp Reference Manual).

The bindings between keys and commands are recorded in tables called keymaps.  See Keymaps.

When we say that ``C-n moves down vertically one line'' we are glossing over a subtle distinction that is irrelevant in ordinary use, but vital for Emacs customization.  The command next-line does a vertical move downward.  C-n has this effect because it is bound to next-line.  If you rebind C-n to the command forward-word, C-n will move forward one word instead.

In this manual, we will often speak of keys like C-n as commands, even though strictly speaking the key is bound to a command. Usually 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 C-n (next-line) moves point vertically down'', meaning that the command next-line moves vertically down, and the key C-n is normally bound to it.

Since we are discussing customization, we should tell you about variables.  Often the description of a command will say, ``To change this, set the variable mumble-foo.''  A variable is a name used to store a value.  Most of the variables documented in this manual are meant for customization: some command or other part of Emacs examines the variable and behaves differently according to the value that you set.  You can ignore the information about variables until you are interested in customizing them.  Then read the basic information on variables (see Variables) and the information about specific variables will make sense.