So I’m guessing that the +4 digits are most handy for sorting for delivery then? By that I mean that if I look up my particular zip+4, it narrows down to six houses in a row on the same side of my street. So it would seem to me that it would be a good way to sort within the truck, or figure out what letters went on which trucks without having to sort the addresses exactly.
Melissa is one of the largest private data collectors out there. They sell commercial feeds back to businesses for analysis.
Nope the last digit of zip code itself is neither 0 nor 6. It’s a 7.
Some good answers in here already.
There are some but not a lot of Zip +4 that are unique addresses. As others have said, a business that generated a ton of volume.
When Zip +4 came out the mail sorting machines would sort it in order. The +4 was a range of addresses in that zip code. These ranges were sorted down to the particular route they were delivered by. When the letter carrier got their mail from the sortation plant they would then ‘case’ their mail. Which is a large metal case with dividers for every single address on their route. In the past they got their mail in Willy Nilly order. It took them awhile to sort it into the proper delivery sequence. The +4 made it much easier to sort.
When they added on the extra 2 digits it was in fact the last 2 digits of your house address. There were special rules for apartments and trailer parks. This allowed the sorting machines to sort in delivery sequence. That meant the letter carriers no longer needed to stand at their case in the morning and sort all of their mail. Huge savings in office times. The letter carriers still had to sort ‘flats’ which are magazines or large envelopes into delivery order. They also had to sort regular envelopes that couldn’t be automated and had been sorted manually at the processing plant.
Of course the letter carriers didn’t love this as it meant more time out pounding pavement and less time in the climate controlled office. Upper management doesn’t care whether they love it or not. Routes were re-adjusted and more mail given to carriers since they weren’t going to be in the office.
All this said it just pertained to the ‘city carriers’ who are the guys you see in uniform. They were mandated to take their presorted mail ‘to the street’. The ‘rural carriers’ had no such mandate. These are two completely separate crafts with their separate unions. The rural carriers were prodded to take their mail to the street but there was no stick. Some chose to take it to the street.
City carriers are hourly employees. Rural carriers are basically independent contractors. They are paid by their route. Many factors but the biggest were miles and volume. They would have their mail counted for 2 weeks and this set their pay.
I retired 5 years ago and back then there was a push on by upper upper management to force rural carriers to start taking their mail to the street instead of hand sorting mail that was already in order. I’m not sure if that is going on now. Their mail makeup has changed so much it’s dramatic. Gone are the days of lots of first class letters. The majority of their mail now is standard mail (junk) and packages which carriers detest.
For more info than anyone asked for: the old barcodes were made up of tall bars and short bars. The tall bars were 1s and the short bars were 0s. The very first bar was called a framing bar. The digits of your zip code were made up of 5 bars. They used a 7 4 2 1 0 weighting for the groups of 5. So if you had a tall short tall short short the number was 9. This followed for every group of 5. Then you had one last number for accuracy. If you added up all the numbers for example 12345-67-89 you come up with 45. The checksum bit or number would be a 5 so it would be short tall short tall short. The checksum bit would be followed by a final tall framing bit.
They later on moved to what was called the 4-State barcode. Not state as in United States as in 4 different state the bar could be in. I honestly can’t remember all the data that can be crammed in that barcode but it was your address and maybe some of the mailer info.
I worked at the USPS as a clerk then an electronics tech and then finished out my last 11years as an Operations Support Specialist. I certainly could have got some of this wrong but hey…I’m 60. I’m lucky I remembered as much as I did.
I don’t think they did inform people - the only mail I’ve ever gotten with zip+4 is mail from businesses that I assume have been using some sort of software/lists to get the zip+4 since 1983.
It wasn’t a big deal for ordinary people - certainly didn’t get the kind of push the original Zip Code launch got in 1963. There may have been a postcard or two from the local Postmaster informing us that our new +4 code was XXXXX-XXXX. Businesses were the original target, since they code pre-sort their mass mailings before the stuff even got to the post office.
This is what I remember too. I don’t remember any major announcements. The USPS just started using +4.
I was an electronics technician for the USPS way back in the mid 80s until 2003 when I moved into management.
Yes, the big mailers put it on themselves. We had machines called MLOCRs aka Multi-Line Optical Character Readers that would read the address and spray the appropriate 9 digit and later the 11 digit barcodes on. So it wasn’t a big deal for the every day consumer.
The early machines could pretty much just read machine printed mail from businesses. As computing power got greater I watched as we went from that to software that could read some of the most chicken scratch handwriting you’ve ever seen. I’d worked in the field of electronics from the time I was 17 in the Marine Corps and witnessed the massive improvement in work they did.
I remember going in on a Sunday and loading the software that was supposed to make this jump on hand written. I took their word for it but was skeptical. It was a several hour software load. When I finished it I went up front and grabbed a handful of mail to run through the machine. I ran it through and it sprayed barcodes on a huge majority. I checked every piece against the database to ensure it sprayed the correct one. It didn’t miss one. I shook my head at just how good it was.
I take no technology for granted. We live in great times as far as that goes.
Yes, each +4 code is one side of a block, and the next +4 code will be the other side of that same block. The carrier will go up one side and then back down the other side. But the next block over won’t usually have the next +4 code – they tend to leave unused +4 codes in case they need to add one in the future.
The +4 codes are tuned to the carriers route. For cul-de-sacs, the same carrier will do all the way around, so all use the same +4 (many don’t have houses on both sides of the street). And there can be gaps in the number sequence, and sometimes, different carriers will do alternate streets in the same area (no idea why).
They didn’t. The Post Office did inform all the mass mailers of this, and fairly soon, started charging extra for bulk mail without +4 codes, and then refused them entirely (must pay 1st class rates). The PO didn’t think average people would have any interest or need to know their +4 code. But now, many people do know it, and use it.
It used to take a lot of technology, like dahbeed described, to deal with zip codes. Now it’s built into MS Word. Word will automatically print the USPO barcode on addresses for you.
What’s annoying now, 36 years later, is websites that can’t accept a valid 9-digit zip code! And they often delete the whole address you entered because they can’t handle the +4 zipcode.
This is what I remember too. No significant announcement to the public.
I started using the +4. There’s enough misrouted mail that I figure I’ll help my mail deliverer as much as possible.
And this, from a former USPS LSM clerk, on Tour 3 where the toughest people work! 
Of course, for a lot of ID numbers we use, the number contains information coded into various subfields of the number and can’t be fully assigned. The OP finding it “surprising” probably depends on the kind of area they are accustomed to thinking of as a zip code. I got curious and looked up the 5 digit zip code containing the highest population. As of 2013 it was 79936 in El Paso, TX. Population > 114K people, which probably translates to something in the vicinity of 30-50K residences (IIRC, the average is about 2.5 people per household, but it’s obviously going to vary a lot between neighborhoods). And on the map, it borders I-10, with a commercial strip alongside it, so there’s probably a lot of businesses, too. The average zip code contains about 7500 people and 200 businesses, but the distribution is very skewed. My source claimed that 10001 in New York City contained the most businesses - over 7200. Unless you continually redraw geographically based districts, they will get skewed as population shifts. The USPS has redrawn zip code boundaries at times, but it is to reflect their operations, not to balance the population numbers.
Delivery Point Sequencing.
Modern letter sorting machines sort the mail for the carrier into trays with the mail in delivery order.
A ZIP+4 is usually (but not always) one tray’s-worth of mail.
I worked on an an automated system that could take 7 complete zip codes of mail and DPS it into ZIP+4 trays, in delivery route order. About 400K letters in 8 hours.
FWIW…if anything: I once got lazy and addressed an envelope with just the name of a large law firm, the city, state and zip+4. It was returned.
My mother once got a letter delivered with just her first name and the zip code. But it was a fairly small city (population 20,000) and she had been an active citizen there for 60 years.
And that was after they had moved mail sorting from the local post office to a larger city about 75 miles away. Before that , it would have only needed her first name to get delivered.
The USPS will generally make an effort to determine a destination for any piece of mail, no matter how vaguely addressed. But it’ll be quicker if you make it easy on them, and quickest if you include the barcode yourself.