TeX is a powerful text formatter written by Donald Knuth; like GNU Emacs, it is free software. The TeX format has several variants, including LaTeX, a simplified input format for TeX; DocTeX, a special file format in which the LaTeX sources are written, combining sources with documentation; and SliTeX, an obsolete special form of LaTeX1.
Emacs provides a TeX major mode for each of these variants: Plain
TeX mode, LaTeX mode, DocTeX mode, and SliTeX mode. Emacs
selects the appropriate mode by looking at the contents of the buffer.
(This is done by invoking the
tex-mode command, which is
normally called automatically when you visit a TeX-like file.
See Choosing Modes.) If the contents are insufficient to determine
this, Emacs chooses the mode specified by the variable
tex-default-mode; its default value is
Emacs does not guess right, you can select the correct variant of
TeX mode using the commands
The following sections document the features of TeX mode and its variants. There are several other TeX-related Emacs packages, which are not documented in this manual:
 It has been replaced by the ‘slides’ document class, which comes with LaTeX.