The felt stops the pieces slipping, so able to store it upright. For once I've been able to move my puzzle without the fear of losing any pieces. I store lots of the loose pieces on the 2 extra mats and have cut some cardboard to fit inside for the pieces that don't fit on the mats, so everything stays in place. Love it . Fits the 1000 piece Holdson puzzles, but not the Ridley ones.