The prepayment of letters is a nice method of paying the shipping costs. But the story has a twist: When franking a letter, money gets lost because of the indifferences in the weighing of letters. To achieve cloudlessness in the post office, there are letter balances. They were invented because the postal reformer Rowland Hill wrote in his diary, that the minister of finance instructed him to develop a machine, which was able to determine the weight of letters. This was the hour of birth of the letter balance.