Major Modes
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Emacs provides many alternative major modes, each of which customizes Emacs for editing text of a particular sort.  The major modes are mutually exclusive, and each buffer has one major mode at any time. The mode line normally shows 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, so that each Emacs command behaves in its most general manner, and each option is in its default state.  For editing text of a specific type that Emacs knows about, 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.  The ones which are changed frequently are TAB, DEL, and LFD.  The prefix key C-c normally contains mode-specific commands.  In addition, the commands which handle comments use the mode to determine how comments are to be delimited.  Many major modes redefine the syntactical properties of characters appearing in the buffer. See Syntax.

The major modes fall into three major groups.  Lisp mode (which has several variants), C mode, Fortran mode and others are for specific programming languages.  Text mode, Nroff mode, TeX mode and Outline mode are for editing English text.  The remaining major modes are not intended for use on users' files; they are used in buffers created for specific purposes by Emacs, such as Dired mode for buffers made by Dired (see Dired), and 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 to make the paragraph commands 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.)

How Major Modes are Chosen
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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 on special text in the file.

Explicit selection of a new major mode is done with a M-x command. From the name of a major mode, add -mode to get the name of a command to select that mode.  Thus, you can enter Lisp mode by executing 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 correspondence between file names and major modes is controlled by the variable auto-mode-alist.  Its value is a list in which each element has this form,

     (REGEXP . MODE-FUNCTION)

or this form,

     (REGEXP MODE-FUNCTION FLAG)

For example, one element normally found in the list has the form ("\\.c\\'" . c-mode), and 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.)  If the element has the form (REGEXP MODE-FUNCTION FLAG) and FLAG is non-nil, then after calling FUNCTION, the suffix that matched REGEXP is deleted and the list is searched again for another match.

You can specify which major mode should be used for editing a certain file by a special sort of text in the first nonblank 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.  Such an explicit specification overrides any defaulting based on the file name.  Note how the semicolon is used to make Lisp treat this line as a comment.

Another format of mode specification is

     -*-Mode: MODENAME;-*-

which allows you to specify local variables as well, like this:

     -*- mode: MODENAME; VAR: VALUE; ... -*-

See File Variables, for more information about this.

When a file's contents begin with #!, it can serve as an executable shell command, which works by running an interpreter named on the file's first line.  The rest of the file is used as input to the interpreter.

When you visit such a file in Emacs, if the file's name does not specify a major mode, Emacs uses the interpreter name on the first line to choose a mode.  If the first line is the name of a recognized interpreter program, such as perl or tcl, Emacs uses a mode appropriate for programs for that interpreter.  The variable interpreter-mode-alist specifies the correspondence between interpreter program names and major modes.

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, the variable default-major-mode specifies which major mode to use.  Normally its 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.

If you change the major mode of a buffer, you can go back to the major mode Emacs would choose automatically: use the command M-x normal-mode to do this.  This is the same function that find-file calls to choose the major mode.  It also processes the file's local variables list if any.