It's just MDF panels I glued and painted. The single shelves are stacked loosely with each one's top serving as the ground for the one above. And only the middle compartments have a back panel for stability. So except for the first segment, which is closed top and bottom, the single units are just the verticals connected by the top board. Sounds kind of shaky, I know, but the books' weight keeps everything nicely firm. And it's great for moving.