According to Andy Peatling: We also decided to go a step further and rework P2 so that it could easily be used as a parent theme and extended with child themes. This will allow theme designers to create awesome new theme designs without having to duplicate all of the existing P2 features. We’ve streamlined the HTML of the theme so that it is much more flexible for creating new designs using CSS. We’ve also added a large number of template based hooks that will allow plugin developers to hook into the theme and output extended functionality.