Using the Salt Lake processing center as an example, here's how a letter gets from one mailbox to another.

Letters are loaded onto a conveyor, where they first are "loosened up" - jostled and shaken, taken uphill and then downhill - so they don't stick to each other. At the end of this part of the route, a spinning drum rests centimeters above the conveyor; it catches oversize and overweight letters and directs them to where they can be processed separately.Lesson No. 1: Don't overstuff an envelope if possible. It will end up having to take a longer route.

Mail that has remained on the main route is zapped through an automated canceling machine that seeks phosphorus on stamps, cancels them and then sorts letters into three groups: those with bar codes along the top or bottom, those with no bar codes and those with no stamp.

Lesson No. 2 - If a stamp has not been placed in the proper position, the canceling machine won't be able to detect it and will kick the letter out, requiring it to be reprocessed.

From the canceling machine, letters with bar codes are sent to a sorting machine that reads the code and sorts the letters by city, ready to be routed to appropriate local carriers or to be trucked or flown to more distant places. The bar-code sorting machine processes up to 40,000 letters an hour and is operated by two people.

Lesson No. 3 - Envelopes that are printed with bar codes are processed in two easy steps and are on their way out of the processing center in the shortest amount of time.

Envelopes without a bar code are sent to an optical character reader, which can read up to six address lines, verify whether the address is correct - it has a directory of all U.S. addresses - and spray a bar code along the bottom right portion of an envelope so it can be sorted by the bar-code machine.

Addresses must be typed, with no punctuation, for the OCR to read them. The OCR reads letters at a rate of up to 36,000 an hour and is operated by two people.

Handwritten addresses and others that can't be read or verified by the OCR are kicked out and sent to one of four letter sorting machines, each operated by 17 people who read the ZIP code and key in its first three digits, routing the envelope to collection bins for general regions.

Letters then are sent through this process again and sorted to cities. Salt Lake branch offices' mail is sorted a final time into carrier routes. Mail headed outside of Salt Lake City must be manually sorted at the destination post office. Each operator handles one letter per second. More than 30,000 letters an hour can be processed this way.

Lesson No. 4 - Hand-addressed letters require five processing steps and are subject to human error, thus increasing the odds that their delivery will take longer.

*****

(ADDITIONAL INFORMATION)

LOST LETTERS

If a letter is torn or damaged during processing, has an address that can't be verified, doesn't have a ZIP code or is undeliverable because of a wrong address, then it it sent to Nixie, a little corner of a mail processing center. There, a postal worker manually places a damaged item in a postal envelope marked with an apology sticker and resends it. Or, if the address is unclear, he tries to locate a correct address or ZIP code in a directory, readdresses the letter and resends it. The worker also will mark the letter for return to sender if a correct address cannot be identified. As a last resort, he sends it to the dead-letter file in San Francisco.