Variable: tex-default-mode
tex-default-mode is a customizable variable defined in tex-mode.el.gz.
Value
latex-mode
Documentation
Mode to enter for a new file that might be either TeX or LaTeX.
This variable is used when it can't be determined whether the file
is plain TeX or LaTeX or what because the file contains no commands.
Normally set to either plain-tex-mode or latex-mode.
Source Code
;; Defined in /usr/src/emacs/lisp/textmodes/tex-mode.el.gz
;;;###autoload
(defcustom tex-default-mode #'latex-mode
"Mode to enter for a new file that might be either TeX or LaTeX.
This variable is used when it can't be determined whether the file
is plain TeX or LaTeX or what because the file contains no commands.
Normally set to either `plain-tex-mode' or `latex-mode'."
:type 'function
:group 'tex)