For Riga Minox to Minox B, the film advances each time the camera is closed, regardless of whether a picture is taken or not. Opening the camera causes the pressure plate to press the film into a concave or flat (depending on the model) surface to stiffen thin emulsions for better clarity. When the camera is closed, the pressure plate moves back from the film plane, thus allowing the film strip to move freely to advance to the next frame. From Minox BL onward, the camera is equipped with a "freewheeling" mechanism, such that the film advances one frame only when a picture is taken, otherwise, closing the camera does not advance a frame.