RockBros Cycling Bag Road Mountain Bike Seat Post Bag Rack Pack Black Quick release system, easily be fixed with seat posts Reflective strap to keep cycling safe Pockets on each side can stretch and extend,larger capacity Adjustable Velcro Enough space: it can fit keys, a mobile phone, multi tools, a wallet and those all important energy snacks to keep you going.Moreover, the outer can be extended for a water bottle.