3 - Box Spring
If you are looking to add a little height to your bed or just simply extend the life of your Beautyrest® mattress, the Beautyrest® Flat Mattress Foundation is the perfect solution. Designed to provide long-lasting comfort, the Beautyrest® flat bed foundation includes a spruce framework and welded steel elements for greater support and durability. 
Available in all standard sizes
Center support extends to the floor for queen or king sizes 
Meets or exceeds State and Federal Fire Retardancy Standards when used either by itself or with an approved Beautyrest® mattress
you must order 2 Twin-XL to make a King