This is a Delta 1 2 in one cropping guide for 5x7/8x10 inch photos. There is a white side and black side to this device. It allows you to view the 5x7 inch format crop or an 8x10 inch format crop. You simply pull in one direction for the viewing of a 5x7 crop or pull the other direction for a 8x10 crop. It is used on either prints or negatives or slides. The black side is normally used when viewing slide films. This device will allow you to see the cropping of the most popular print sizes 5x7 or 8x10 BEFORE you run off to your darkroom and make prints from your films!