Major Modes
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Emacs has many different major modes, each of which customizes Emacs for editing text of a particular sort.  The major modes are mutually exclusive;  at any time, each buffer has one major mode.  The mode line normally contains the name of the current major mode in parentheses. See Mode Line.

The least specialized major mode is called Fundamental mode.  This mode has no mode-specific redefinitions or variable settings.  Each Emacs command behaves in its most general manner, and each option is in its default state.  For editing any specific type of text, such as Lisp code or English text, you should switch to the appropriate major mode, such as Lisp mode or Text mode.

Selecting a major mode changes the meanings of a few keys to become more specifically adapted to the language being edited.  TAB, DEL, and LFD are changed frequently.  In addition, commands which handle comments use the mode to determine how to delimit comments. Many major modes redefine the syntactical properties of characters appearing in the buffer.  See Syntax.

The major modes fall into three major groups.  Programming modes (see Programs) are for specific programming languages.  Text modes (like Nroff mode, TeX mode, Outline mode, XML mode, etc.) are for editing human readable text.  The remaining major modes are not intended for direct use in editing user files; they are used in buffers created by Emacs for specific purposes. Examples of such modes include Dired mode which is used for buffers made by Dired (see Dired), Mail mode for buffers made by C-x m (see Sending Mail), and Shell mode for buffers used for communicating with an inferior shell process (see Interactive Shell).

Most programming language major modes specify that only blank lines separate paragraphs.  This is so that the paragraph commands remain useful. See Paragraphs.  They also cause Auto Fill mode to use the definition of TAB to indent the new lines it creates.  This is because most lines in a program are usually indented.  See Indentation.

Choosing Major Modes
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You can select a major mode explicitly for the current buffer, but most of the time Emacs determines which mode to use based on the file name or some text in the file.

Use a M-x command to explicitly select a new major mode.  Add -mode to the name of a major mode to get the name of a command to select that mode.  For example, to enter Lisp mode, execute M-x lisp-mode.

When you visit a file, Emacs usually chooses the right major mode based on the file's name.  For example, files whose names end in .c are edited in C mode.  The variable auto-mode-alist controls the correspondence between file names and major mode.  Its value is a list in which each element has the form:

     (REGEXP . MODE-FUNCTION)

For example, one element normally found in the list has the form ("\\.c$" . c-mode). It is responsible for selecting C mode for files whose names end in .c.  (Note that \\ is needed in Lisp syntax to include a \ in the string, which is needed to suppress the special meaning of . in regexps.)  The only practical way to change this variable is with Lisp code.

You can specify which major mode should be used for editing a certain file by a special sort of text in the first non-blank line of the file. The mode name should appear in this line both preceded and followed by -*-.  Other text may appear on the line as well.  For example,

     ;-*-Lisp-*-

tells Emacs to use Lisp mode.  Note how the semicolon is used to make Lisp treat this line as a comment.  Such an explicit specification overrides any default mode based on the file name.

Another format of mode specification is:

     -*-Mode: MODENAME;-*-

which allows other things besides the major mode name to be specified. However, Emacs does not look for anything except the mode name.

The major mode can also be specified in a local variables list. See File Variables.

When you visit a file that does not specify a major mode to use, or when you create a new buffer with C-x b, Emacs uses the major mode specified by the variable default-major-mode.  Normally this value is the symbol fundamental-mode, which specifies Fundamental mode.  If default-major-mode is nil, the major mode is taken from the previously selected buffer.

Mode Hook Variables
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The last step taken by a major mode, by convention, is to invoke a list of user supplied functions that are stored in a ``hook'' variable. This allows a user to further customize the major mode, and is particularly convenient for setting up buffer local variables (see Locals).

The name of the hook variable is created by appending the string -hook to the name of the major mode.  For example, the hook variable used by text-mode would be named text-mode-hook. By convention the mode hook function receives no arguments. If a hook variable does not exist, or it has the value nil, the major mode simply ignores it.

The recommended way to add functions to a hook variable is with the add-hook function.  For example, to automatically turn on the Auto Fill mode when Text mode is invoked the following code can be used in the initialization file (see Init File)

     (add-hook 'text-mode-hook 'turn-on-auto-fill)

The add-hook function will check that the function is not already listed in the hook variable before adding it. It will also create a hook variable with the value nil if one does not exist before adding the function. add-hook adds functions to the front of the hook variable list. This means that the last hook added is run first by the major mode. It is considered very poor style to write hook functions that depend on the order that hooks are executed.

Hooks can be removed from hook variables with remove-hook.