Bicycle Aluminum Rear Double Water Bottle Cages This is an aluminum alloy double water bottle holder, it is mounted below the bike rear seat.It can be installed two single bottle cage, and very lightweight,so you could carry more water on your longer rides. It has four colors:black,silver,blue,red. Not compatible with i-beam type saddle. Standard dual rail type only.Use for road bikes, mountain bikes, TT. VERY EASY TO INSTALL--Coming with 6 screws, installed in the backseat, extend installation of two water bottle cages, fits for long time riding or Long distance travel. Never worry about no water in halfway. NOTE--This specific item is only the adapter so you need to either already have the cages or purchase them with this item.