Abstract: A binder mechanism having two longitudinal bars on which the page holding prongs are mounted. The bars are interconnected by two rectangular movable hinge members situated proximate opposite ends of the bars. The hinge members which have two right angle sections interconnected by a piano hinge are rotatably connected to the bars and define a rectangular box-shaped structure with the bars when the binder is in the locked position. Both hinge members and bars have a thin rectangular cross-section, the long dimension of which is vertically oriented when the hinge members and the bars are in the locked position. The hinge members pivot about pins which interconnect the hinge members to the bars to rotate to an open position with the piano hinge rotated 90.degree.. One hinge member then rotates about the piano hinge to open the binder so that the longitudinal bars are in a horizontal position and the prongs are vertically disposed. In this position, pages can be removed and inserted.