Canceling marks a stamp with indelible ink, making it unusable. Initially, these markings were applied by a clerk using a handstamp, but humans could not keep up with the ever-increasing volume of mail. As early as 1862, inventers began experimenting with machines to cancel the mail mechanically. Thomas and Martin Leavitt of Boston patented the first practical canceling machine in 1876, and the next year the Post Office Department put in an order for one hundred machines. In a speed trial, the Leavitt machine cancelled 25,000 postal cards in an hour, compared to an experienced clerk’s rate of 1,500 to 2,000 an hour. Today, almost all mail is machine-cancelled.
Fundamentally, all canceling machines share basic characteristics. Feed rollers separate and introduce a piece of 'faced mail' into the machine. A trip mechanism detects the leading edge of the mail piece and actuates the rotation of a canceling die hub and ring die. After the impression is made on the mail piece, it passes to a stacker in which mail accumulates until removed by the machine operator.
The die hub may have wavy-bars or a 'slogan' that is aligned to place a cancellation over the postage stamps on a mail piece. A slot on the die hub holds the 'ring die'. This engraved steel block is mortised to hold changeable month, date, time, and year type in its center. Around this opening and usually within a circular border, the town name, state, and, if applicable, a zone, or ZIP Code appear. These are the postmark that appears to the left of the cancellation.
Scheele, Carl H. A Short History of the Mail Service. Smithsonian Institution Press: Washington, DC, 1970.