How are one-sidebar sidebar-second classes set?

Thanks for the incredibly helpful project. We have a working site up and running with a renamed basic theme.

I was just wondering how the sidebar classes are set in the body element of the page?

I've been looking at template.php - and don't see a function there that's doing this (unless I'm missing this - although I see functions for role classes and others).

Would be really interested to know how classes like one-sidebar sidebar-second etc. are set in the body element, since this is what makes this theme so powerful, combined with the grid system for dynamically changing grid elements and the layout based on how many sidebars are present, and which one if only one.

Any tips or pointers greatly appreciated.