I've been playing around with the CSS for the site, and found an annotated WP layout file on Nuclear Moose's website. It's a handy reference, but I think it would be more useful if the commentary were actually enclosed in CSS comments. To that end, I've copied the relevent text and enclosed the ex­pla­na­tions in comments. Here's my commented version. Hope this helps someone get a handle on layout in WP (I know it's taking me long enough).