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
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
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,
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:
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
nil, the major mode is
taken from the previously selected buffer.