Rolltop closure means it can be big or small; and this bag can get really big! It's great for the minimalist roadie, the over-prepared rider, and everyone in between. Four possible modes are shown here but as an example, you can fit a 29er tube, a repair kit, snacks, and extra layers. Even maxed out, this bag is super secure thanks to the cam lock buckle and seatpost strap. Easy installation as shown below.