Why are some .tex files opened in Vim as filetype=plaintex and others as filetype=tex? Since the same extension, .tex in this case, is used for multiple filetypes, "Vim tries to guess what kind of file it is" according to the Vim filetype-overrule documentation.
The ft-tex-plugin section of the Vim filetype documentation lists the rules used by Vim (versions 7 and higher) to determine which filetype to use for .tex files:
- If the first line of the file is
%&<format>where<format>can beplaintex,context, ortex, then the filetype is set to plain TeX, ConTeXt, or LaTeX, respectively. - If there is no format designator command on the first line, then Vim searches the file for keywords to determine if the filetype should be set to
context(ConTeXt) ortex(LaTeX). - If no keywords are found, the filetype is set to
plaintex(plain TeX).
The last rule is important. If you create an empty .tex file using a command like touch myfile.tex, then when you open it in Vim the filetype will default to plaintex, since the file is blank.
You can change the default behavior by setting the global variable tex_flavor in your .vimrc:
let g:tex_flavor = "plain"let g:tex_flavor = "context"let g:tex_flavor = "latex"