©2011 Smithsonian Institution and the United States Postal Service
When someone sends a letter, it enters a system already at work that most people never get to see.
The United States Postal Service processes hundreds of millions of mail pieces everyday.
The Postal Service delivers almost half the world's mail to over a hundred and fifty million addresses, through a network of thousands of post offices.
These offices are supplied by a network of hundreds of processing and distribution centers around the nation.
The Postal Service separates mail into three categories: letters, flats, and packages.
Small pieces of mail like letters, bills, and post cards are all processed by the same set of machines.
Processing letters begins with culling, or filtering out mail that cannot be handled by machines down the line due to size, shape, or weight.
Letters then enter the Advanced Facer-Canceller system.
This machine uses specialized cameras to take pictures of envelopes as they speed by.
These pictures are used by the computer to find the stamp, locate the address, read the handwriting, and compare the address against a database of known addresses.
It faces the letter in the right direction, sprays it with the unique ID tag and cancels the stamp with a postmark.
The letters are then transferred to the Delivery Barcode Sorter.
Postal workers feed the letters into this machine by hand.
This machine sorts letters into "delivery point sequence," or the order that postal carriers will deliver them along their routes.
After letters are sorted, they are moved to the loading dock.
Customers often bring large bundles of magazines to distribution centers for processing.
The Postal Service refers to magazines, catalogs, and similar items as "flats."
Large bundles of flats must be weighed and verified before they are processed.
They are then taken to a preparation area.
There, they are separated and ready for processing.
After they are prepared, bins of flats go into the Flats Sequencing System, a machine the length of a football field.
The flats travel along a conveyor system to a feeder, where they are removed from the bins and sent one by one to the scanning system.
A high-speed camera captures images of flats to identify their delivery addresses.
A computer interprets the scanned addresses and sends sequencing information to the machine's robotics system.
The flats are then sorted into delivery order for postal carriers.
Sorted flat so then transfers to trays and automatically loaded onto carts.
They are then moved to the loading dock.
Packages can be particularly difficult to process by machine because they come in a wide variety of sizes and shapes.
The Automated Package Processing System is uniquely equipped to deal with this kind of mail.
Packages are spread out as they move along a series of belts and rollers.
As the packages enter the scanning and imaging tunnel, the machine reads their addresses.
It determines the package dimensions... and weight, checks for proper postage, and scans the barcode, updating the package's tracking information.
The packages then travel along a conveyor, before being kicked off into bins by destination.
Packages are then moved to the loading dock, where they are loaded onto trucks along with letters and flats going to the same post offices.
As morning approaches, drivers deliver the sorted mail to the appropriate post offices.
After the mail arrives, postal workers separate it for pickup.
Carriers gathered the sorted letters, flats and packages to take out on their routes.
Mail delivery connects people and businesses all across the country.
Everyday technology keeps mail flowing through this constantly moving network.