Abstract: A base which is adapted to hold a pile of similarly dimensioned sheets includes a substantially flat and rectangular holder, which defines two parallel and normally horizontally positioned sides, and a frame which may be attached to, and surround the holder. The holder is formed with normally upright rim strips, and the frame covers the rim strips. Each rim strip is formed with a guide slot; a bar of transparent material is disposed in parallel with the horizontally positioned sides, and is slidable along the holder in a direction substantially perpendicular to the horizontally positioned sides. The frame is formed with recesses in the region of the guide slots, so as to accommodate the slidable bar, and each recess in the frame, and each guide slot corresponding thereto is located in a center region of a corresponding rim strip, and extends along about one third along a length thereof.