The NiSi 77mm Metal Stack Caps are a set of top and bottom durable metal caps to fit 77mm filters. Simply screw your filters together then screw the stack caps on the top and bottom. TheNiSi 77mm Metal Stack Caps can be used with a minimumof 1 x 77mmfilter and can be used to protect 20+ 77mm filters screwed together. Themetal construction offers protection to filter surfaces andprotects the filters from dust, scratches and fingerprints while not in use. The Stack Capsenable a more organized, streamlined system for storing and carrying your circular filters.