International Top Indentation
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 that
are changed frequently are
DEL, and C-j. 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.
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), 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
TAB to indent the new lines it creates. This is
because most lines in a program are usually indented.