How Does the Post Office Sort the Mail?

Step 1

Packages are sorted from envelopes.

Packages are handled by a conveyor belt system and simply routed to the appropriate post office where the postal carriers sort them by hand.

Step 2

Remote Barcode Cancellation System (RBCS) and Input SubSorting (ISS)

Does the following:

1. Mail enters on a conveyor belt where letters are placed horizontally. Square envelopes cause a problem because the machine cannot determine if the envelope is facing side ways. Square envelopes have a surcharge to their postage because they require special handling.
   
   Envelopes are flipped, if necessary, so that all addresses are facing one side. The address can be either right-side-up or up-side-down.
   
   
2. The stamp is cancelled.
   
   
3. A fluorescent orange bar code is applied to the back of the envelope which uniquely identifies each piece of mail. This orange code is so long and unique that it will not be repeated for at least 30 days.
   
   
4. An image of the front of the envelope is taken. It is matched with the orange bar code and stored in a computer for 30 days.
   
   
5. If a company has contracted with USPS to do so, they will be notified that this piece of mail has come through the processing center.

Step 3

The PostNET Code

Address Decoder or Image Processing Sub-System (IPSS)

1. Every envelope needs an 11 digit bar code (PostNET Code) printed on the front. This code is unique to the delivery point. It represents the nine digit zip code plus a two digit destination code.
   
   If the envelope already has a pre-printed PostNET code on the front, nothing is done. Organizations are charged a discounted postage rate if this code is present.
   
   
2. If the envelope does not have a PostNET code on the front, this machine reads the address with an OCR and computes the PostNET code. The 5 or 9 digit zip code is used as a cross check of the address.
   
   
3. If the computer cannot determine the address with confidence or if the zip code does not match, an image of the address will be shown to a postal employee at a computer terminal in Utah. The operator will read the address and type it into the computer. As soon as the computer has enough information, it skips to the next one.
   
   
4. If a change of address is in the system, the new address is noted, along with the PostNET code. A sticker with the new address is applied.

When this step is finished, every piece of mail will have a PostNET code assigned to it.

Step 4

Output Sorting (OSS)

1. The orange bar code on the back of the envelope is matched with the computer record. The PostNET code is printed on the front of the envelope.
   
   
2. If a forwarding address has been noted, a label with the new address is applied along with the new PostNET code.
   
   
3. The mail is sorted by 3 or 5 digit zip code, depending on how many destination addresses are in that code.

Step 5

In this step, the mail for a particular zip code is sorted by address. For an explanation of how this sorting works, see a simplified sort example.

The sorter has 220 output buckets. Mail is run through the sorter twice, allowing it to be sorted into 220² or 48,400 different delivery addresses.

Mail is sorted using the unique 11 digit PostNET code. On the first pass, output bucket number 1 will have mail for addresses 1, 221, 441, 661, 881, etc.

Output bucket #2 will have mail for addresses 2, 222, 442, 662, 882, etc.

Output bucket #3 will have mail for addressed 3, 223, 443, 663, 883, etc.

Output bucket #4 will have mail for addressed 4, 224, 444, 664,884, etc.

And so on.

Then the mail is run through the sorter a second time. Each bucket is run through IN ORDER. Input bucket #1 has all the mail, randomly, for the first address in each of the output buckets (1, 221, 441, 661, 881, etc.). The mail is deposited in the proper output bucket. All mail for address 1 goes in the first bucket, 221 goes in the second bucket, 441 goes in the third bucket, 661 goes in the fourth bucket, 881 goes in the fifth bucket, etc.

Bucket #2 has all the mail, randomly, for the second address in each output bucket (2, 222, 442, 662, 882, etc.) All mail for address 2 goes in the first bucket, 222 goes in the second bucket, 442 goes in the third bucket, 662 goes in the fourth bucket, 882 goes in the fifth bucket, etc.

Bucket #3 has all the mail, randomly, for the third address in each output bucket (3, 223, 443, 663, 883, etc.) All mail for address 3 goes in the first bucket, 223 goes in the second bucket, 443 goes in the third bucket, 663 goes in the fourth bucket, 883 goes in the fifth bucket, etc.

And so the process continues until the mail has been sorted in order and is ready for the carrier to deliver.