<p><a href="http://meta.stackexchange.com/questions/4798/lets-make-it-easier-to-reply-within-comments/35917#35917">From meta.stackoverflow</a>:</p>

<blockquote>
  <p>Rules:</p>
  
  <p>Only applies to other people in the comments that you are commenting on, or editors of the current post.</p>
  
  <ol>
  <li><p>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).</p></li>
  <li><p>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.</p></li>
  <li><p>Spaces cannot be used to match, so
  if the person's display name is
  "Peter Smith" then just use @peter
  or @petersmith to match.</p></li>
  <li><p>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.</p></li>
  </ol>
</blockquote>