This is where our Logo and Site info goes. Maybe the "Logon/Logoff/Join" button as well.
In this layout the side column widths are in pixels and the centre page adjusts in size to fill the rest of the screen. Vertical dimensions are left unset so they automatically stretch to the height of the content. Layouts that use pixel widths are great for images because you can make them fit perfectly within a column without gaps.
I've colour coded each div so it's easy to see: