Slang term that refers to a preliminary rough calculation, plan or draft. It suggests something written on any convenient medium, like an envelope or cocktail napkin, but that requires further thought and refinement – the beginnings of a plan or idea, but not something fully worked through. However, famously, Wayne Pfaff’s back of the napkin design for a ZIF socket was found to be a violation of the on-sale-bar in Pfaff v. Wells because his customer, Texas Instruments had enough confidence in Pfaff’s abilities to see it as the basis to order the product.