More and more theme structures are based on the 960Grid System created by Nathan Smith (http://960.gs/). The 960Grid System is a way to streamline interface design by providing commonly used dimensions, based on a width of 960 pixels.
Thank you for your interest. I apologize for any inconvenience this limited access causes. This online guide is now a book called Drupal: The Guide to Planning and Building Websites. The content of the guide is being updated, expanded, and edited. It will include tips for D6 and D7. I hope you will find it useful.
