Mysterious Orange Marks

Q: What are those little orange marks on the back of the envelopes I get in the mail? If they encode my address I'd expect to see the same marks on all letters, but they always seem to be different.
--John G., Albuquerque, NM

A: You're right: the marks are different because they merely name the letter, like "Joe". Each letter gets a different name. That way, the system can find a particular letter, Joe, again if it needs to.

Why should it? Ah, we're getting to the real problem: envelopes must have a zip code that machines can read: the black bar code. Our automated mail system produces the bar code by an intricate procedure.

I called the bar code expert at the Albuquerque post office, Anthony Baca, to find out what's going on.

"A machine reads your address and (if it can decipher the zip code) prints a corresponding bar code on the envelope but prints the orange code first in case the machine fails to read the address," he said.

"What?!!"

I got a tour of the post office to find out.

The envelopes, on a conveyor belt, approach an incredibly fast reader machine, called an advanced Optical Character Recognition (OCR) machine. The letters zip into this machine in a white blur-- 9 envelopes a second, 33,000 envelopes an hour. Once there, the machine reads the characters of each address, takes a picture of the address, sprays each envelope with its orange-mark name tag, and sends the letter out on the belt a certain distance to give itself time to finish its job.

The letter goes that distance and returns while the machine determines if it can recognize the characters in the envelope's address. If it succeeds for a given letter, the machine puts the correct bar code on the envelope. If it fails, that letter gets special treatment. The smart machine does not fail often since it can decipher even hand printed letters if the printing is in all caps.

When the OCR machine fails, it needs a human to read the address. All the address-reading humans are in central locations to save money. It costs time and money to send the physical letter. So the system does the operation by phone and refers to the letter by its orange marks, Joe, in our example.

The machine sends Joe's picture (showing the address) and Joe's name to a regional office, perhaps Utah, via dedicated phone lines. The Salt Lake City office employs 800 people to read addresses when the local OCR machines fail.

A Utah computer answers the phone, receives the picture, and displays it to a human. The human reads your address and types it into the computer. The machine looks up the corresponding zip code--complete with a 4-digit route code, and the last two numbers of your street address--and determines the correct bar code.

Utah phones the data back to the original post office. The OCR machine there reads the orange name tags of pending letters, finds Joe again, and prints the bar code on the envelope. Done.

(Answered Nov. 1, 2002; updated Oct. 10, 2007)