I just stumbled over a global variable called
$content_width. It was new to me, so far I could see that it's used in themes, both the standard twenty ten one and third party ones.
It looks like that it contains the width of the content area in pixels. Since global variables are not documented in the codex, I had trouble finding information about it.
When was the
$content_width global variable introduced and for what reason? Is there a ticket related to it?