The EZ Mount Triple Stack and EZ Mount Quad Stack are an extensions of our original suction cup window sill beds. They come in two different designs. The EZ Mount Triple Stack offers 3 levels of playing and bird watching for kitties, while the EZ Mount Quad Stack has an added 4th shelf. Both designs have holes for climbing through to explore each level for the best view. They can be installed on a sliding glass door or a big picture window. No tools are necessary. Two push n' turn suction cups plus giant suction cups for each shelf provide ultimate strength. Exclusive design allows both products to be folded up so window blinds and drapery can function normally. One year limited warranty.