How Is Mail Sorted?
At large Post Office sorting offices the mail is separated mechanically by a letter segregator. Thick packets and envelopes are flipped down a chute for sorting by hand, while the rest of the mail is moved along to a machine called an automatic letter facer. The facer arranges the letters so that their stamps are all in the same position and then puts a postmark on each of them showing the place and date of posting. This process, known as cancelling, can be carried out at a rate of about 20,000 letters an hour.
Letters then go to the letter-coding desk, where operators read the post codes and tap them out on a keyboard. Two rows of dots are printed; one above and the other below the address, the top row denoting the district to which the letter is to be delivered and the lower row the name of the street. Although the dots are almost invisible to the eye, they are detached by the automatic serving machine, which finally sorts the letters ready for delivery.
A sorting office or processing and distribution center (P&DC; name used by the United States Postal Service (USPS) is any location where postal operators bring mail after collection for sorting into batches for delivery to the addressee, which may be a direct delivery or sent on wards to another regional or local sorting office, or to another postal administration.
Most countries have many sorting offices; the USPS has about 275. Some small territories such as Tahiti have only one. Sorting vans were used at various times; the UK had sorting vans, or carriages, in their Travelling Post Offices but those services were terminated in 2004. While in the USA the Railway Mail Service used a Railway post office for sorting the mail. As of 2017, Germany has about 95-98 sorting offices across the country.
The United Kingdom Royal Mail’s Mount Pleasant Sorting Office was the world’s largest sorting office at the beginning of the 20th century but is now only the largest one in London. Military mail systems, such as the British Forces Post Office and U.S. Military Postal Service, have their own dedicated sorting offices.
For much of the 20th Century, mail was sorted by hand using what is called a “pigeon-hole message box” method. Addresses were read and manually slotted into specific compartments. While early forms of a mechanical mail sorter were developed and tested in the 1920s, the first sorting machine was put into operation in the 1950s.
To handle rapidly growing mail volumes, the United States Postal Service installed the first semiautomatic sorting machine on April 10, 1957. The Transorma Letter Sorting Machine, manufactured by the Dutch company Werkspoor and distributed in the United States by Pitney Bowes, consisted of an upper and lower section, a conveyor belt transport and a series of five sorting keyboards.
Operators read the destination and keyed a sorting code. The letter was then automatically transferred to a letter tray and deposited into one of 300 chutes. The Transorma could sort 15,000 letters per hour, double the amount that the same number of clerks could do by hand.
In 1965, the Postal Service put the first high-speed optical character reader (OCR) into operation that could handle a preliminary sort automatically. And in 1982, the first computer-driven single-line optical character reader was employed – which reads the mailpiece destination address then prints a barcode on the envelope that could be used to automate mail sorting from start to finish.
With the U.S. Postal Service introduction of postal worksharing, ZIP + 4 and the POSTNET barcode in 1980, companies were given an incentive to sort their mail prior to inducting it at the Post Office. Today, presort and automation discounts can save companies up to 50% or more on postage—and many companies use Mail Sorters to sort both incoming and outgoing mail.
Mail sorters can process up to 55,000 #10 envelopes per hour. Systems can scan and archive mail piece images during the sort process for compliance and proof of mailing. Multi-Line Optical Character Reader (MLOCR) technology can also read and validate both machine-print and handwritten pieces. Companies who receive a high-volume of incoming mail, including remittance checks, orders and correspondence, use mail sorters to help ensure that mail reaches the right person or department quickly and efficiently.
Recent innovations allow for mixed mail sorting—providing for postal discounts on letters, small parcels, flat mailers, irregularly shaped pieces, padded envelopes and even Polywrap sheets. In 2007, the USPS introduced Shape-Based Pricing which offered more significant postal discounts for mailers who sorted flat-sized mail pieces. In response to this postal change, the market responded with new low-cost systems designed specifically to support flat mail sorting for mailers who process between 500 and 10,000 First-Class flats per day.