Past-due customer prodding tool, urging the laggard to make a payment. Typically issued one after another. Tone and language get more direct (testy?) as additional letters go out, unless some money is paid. Maintaining the customers’ goodwill that was painstakingly acquired over the years, while securing past-due amounts is a key point in a good letter. Also known as dunning letter or dunning notice. Contrast with collection letter, which is different.