div#content, just before the div for each post.
If more than one post is shown on a page, this hook fires before each of them. The position of the post is passed as a parameter if needed in actions added to this hook (targeting only the first or third posts, for example).
<div id="content" class="hfeed"> thesis_hook_before_post_box <div class="post-1 post hentry category-uncategorized post_box top" id="post-1">
There are no default functions added to this hook.