[From meta.stackoverflow][1]:

>Rules:
>
>Only applies to other people in the comments that you are commenting on, or editors of the current post.

>  1. Response must include @username that
>     you are referring to, where
>     "username" is a reasonable match to
>     the user's current display name (as
>     seen in the comments above yours).
>     
>  2.    There must be a starts-with, case
>     insensitive match of at least THREE
>     characters to the displayname. So @a
>     and @ab will never match anyone or
>     anything.
>     
>  3.  Spaces cannot be used to match, so
>     if the person's display name is
>     "Peter Smith" then just use @peter
>     or @petersmith to match.
>     
>  4. Matching is performed in reverse
>     chronological order, so if there are
>     five people named "John" in the
>     comments, writing "hey @john, have
>     you considered apples?" will match
>     the most recent John to comment.

  [1]: http://meta.stackoverflow.com/questions/4798/lets-make-it-easier-to-reply-within-comments/35917#35917