Mounts to interior glass patio doors and large windows and offers multiple levels that offer your cat fun and comfort with a bird's eye view. 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.