There should be a plate on the front of the enlarger head that gives you the information regarding what condensers and lens you need for the film size you are enlarging. The pair of 240 condensers are designed to be used for 4"x6" and 5"x7" film with a 180mm or 210mm lens. If you were to enlarge a 6x6cm negative you should have a 200 condenser on top of a 130 and use a 80mm lens. In all cases the convex surfaces should be facing each other.