Sometimes it’s nice to have an extra script font available to make distinctions between different kinds of objects clearer. One such font is Ralph Smith’s Formal Script (see here for the TeX Catalogue entry) which is provided by the mathrsfs package.

This gives you a new LaTeX font command \mathscr{} for use in math mode. My version of AUCTeX for Emacs did not include a font key command for this, so I added one (bound to the standard font command prefix -- which for me is C-c C-f -- plus C-x).

(add-hook 'LaTeX-mode-hook (lambda ()        
    (add-to-list 'LaTeX-font-list '(?\C-x "" "" "\\mathscr{" "}"))))

And this is how it looks: