 To create a billing copy, a sheet of heat sensitive paper was inserted between the backing and the entry sheet and passed through the ThermoFax machine, the Model 47 being the most commonly used.As copying technology advanced, Thermofax machines were subsequently marketed as a method of producing transparencies (viewgraphs) for overhead projector presentations.A sheet of heat-sensitive clear stock was placed on top of the original, and passed through a ThermoFax, producing a black image on the clear stock.