This set contains a slide-in sandwiching mount which fits into the Cokin holder. The slide-in mount accepts any of the included 10 pre-shaped black masks. These masks come in the shape of a circle, a keyhole, a heart, intersecting twin circles and an oval (two of each shape; one large, one small). As the black part of the mask blocks out the light and the center does not, the resulting photograph will be framed in that particular shape. The size and abruptness of the transition is affected by the focal length and aperture of your lens.
The slide-in mount included with this set measures 66 x 72mm and fits the Cokin A series holder.