Helps secure books and items on open shelves from falling. One box will do 3 shelves! Simply slide the cord into one end-cap connector and snap together. Cord stretches to remove books! Simply slide the cord into one end-cap connector and snap together. Do this on both sides of the cording and then fasten into the edge of shelf. This flexible cord is made to last and it"s easy to use. Simply stretch the cord to remove an item and let it pop back into place. .