23.1 Major Modes
Every buffer possesses a major mode, which determines the editing behavior of Emacs while that buffer is current. The mode line normally shows the name of the current major mode, in parentheses (see The Mode Line).
The least specialized major mode is called Fundamental mode. This mode has no mode-specific redefinitions or variable settings, so that each Emacs command behaves in its most general manner, and each user option variable is in its default state.
For editing text of a specific type that Emacs knows about, such as
Lisp code or English text, you typically use a more specialized major
mode, such as Lisp mode or Text mode. Most major modes fall into
three major groups. The first group contains modes for normal text,
either plain or with mark-up. It includes Text mode, HTML mode, SGML
mode, TeX mode and Outline mode. The second group contains modes
for specific programming languages. These include Lisp mode (which
has several variants), C mode, Fortran mode, and others. The third
group consists of major modes that are not associated directly with
files; they are used in buffers created for specific purposes by
Emacs. Examples include Dired mode for buffers made by Dired
(see Dired, the Directory Editor), Message mode for buffers made by C-x m
(see Sending Mail), and Shell mode for buffers used to communicate
with an inferior shell process (see Interactive Subshell).
Usually, the major mode is automatically set by Emacs, when you
first visit a file or create a buffer (see Choosing File Modes). You
can explicitly select a new major mode by using an M-x command.
Take the name of the mode and add -mode to get the name of the
command to select that mode (e.g., M-x lisp-mode enters Lisp
mode). Since every buffer has exactly one major mode, there is no way
to “turn off” a major mode; instead you must switch to a different
one.
The value of the buffer-local variable major-mode is a symbol
with the same name as the major mode command (e.g., lisp-mode).
This variable is set automatically; you should not change it yourself.
The default value of major-mode determines the major mode to
use for files that do not specify a major mode, and for new buffers
created with C-x b. Normally, this default value is the symbol
fundamental-mode, which specifies Fundamental mode. You can
change this default value via the Customization interface (see Easy Customization Interface), or by adding a line like this to your init file
(see The Emacs Initialization File):
(setq-default major-mode 'text-mode)
If the default value of major-mode is nil, the major
mode is taken from the previously current buffer.
Specialized major modes often change the meanings of certain keys to
do something more suitable for the mode. For instance, programming
language modes bind TAB to indent the current line according to
the rules of the language (see Indentation). The keys that are
commonly changed are TAB, DEL, and C-j. Many modes
also define special commands of their own, usually bound to key
sequences whose prefix key is C-c (see Keys). Major modes
can also alter user options and variables; for instance, programming
language modes typically set a buffer-local value for the variable
comment-start, which determines how source code comments are
delimited (see Manipulating Comments).
To view the documentation for the current major mode, including a
list of its key bindings, type C-h m ( describe-mode).
See Other Help Commands.
Every major mode, apart from Fundamental mode, defines a mode
hook, a customizable list of Lisp functions to run each time the mode
is enabled in a buffer. See Hooks, for more information about
hooks. Each mode hook is named after its major mode, e.g., Fortran
mode has fortran-mode-hook. Furthermore, all text-based major
modes run text-mode-hook, and many programming language modes
10 (including all those distributed with Emacs) run
prog-mode-hook, prior to running their own mode hooks. Hook
functions can look at the value of the variable major-mode to
see which mode is actually being entered.
Mode hooks are commonly used to enable minor modes (see Minor Modes). For example, you can put the following lines in your init file to enable Flyspell minor mode in all text-based major modes (see Checking and Correcting Spelling), and ElDoc minor mode in Emacs Lisp mode (see Emacs Lisp Documentation Lookup):
(add-hook 'text-mode-hook 'flyspell-mode)
(add-hook 'emacs-lisp-mode-hook 'eldoc-mode)
Footnotes
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More specifically, the modes which are “derived” from
prog-mode (see Derived Modes in The Emacs Lisp
Reference Manual).