Material: high-density anti-tear and water-resistant nylon material with the characteristic of the light weight, good water resistance and tear resistance. And the high density has greatly improved its wear resistance. Inter bag is filled with thick sponge, comfortable, decompression which makes good ventilation effect and provide the best cooling system to keep back dry. Functional design: unique top tripod fastener equipment, internal Velcro combination, free assembly, which is applicable to a variety of models.