however, by default in emacs, there is a command called ‘comment-or-uncomment-region’ that is bound to ‘M-;’ . if emacs recognize a syntax or the mode of the buffer, the correct comment character will be inserted or removed. if emacs doesn’t know the default, it will ask u, and u can insert it in. for example, in .conkerorrc, there is no default. i will put ‘//’ when emacs asks me, and i can use ‘M-;’. very cool!