33:  How do I make Emacs use a certain major mode for certain files?
  
  If you want to use XXX mode for all files which end with the extension
  `.YYY', this will do it for you:
  
    (setq auto-mode-alist (cons '("\\.YYY\\'" . XXX-mode) auto-mode-alist))
  
  Otherwise put this somewhere in the first line of any file you want to
  edit in XXX mode:
  
    -*-XXX-*-