Any postal old-timers around? Review my theory on postal zone numbers

The underlying question is how ZIP codes in Chicago and other large cities were assigned. For the most part, most Dopers would know, ZIP codes in large cities just added the sectional center prefix (606) in front of the zone numbers (40, for example) that had been introduced during World War II because so many experienced postal workers had gone into the service.

But how were the original zone numbers assigned in 1943? My theory is that they were based on the sorting cases already in use. As a city like Chicago grew, additional branch post offices and carrier stations were established. The arrangement of slots in the sorting case used for sending letters to the other branch offices would have been standardized among offices and infrequently changed, as that would slow for weeks the work of trained clerks who already knew the scheme.

In Chicago, the lowest numbers are for the downtown area, followed mostly by areas where branches would have existed prior to 1900. The larger numbers look like they could well have been added later, as new branches were established, and those new offices were simply assigned slots at the end of the scheme/bottom of the case. Thus, when zone numbers were introduced, they simply numbered the slots in order.

(In suburban and rural areas where each town has its own ZIP code, it looks like the last two digits were assigned in rough alphabetical order, though there were many exceptions as well as later additions and splits.)

I don’t suppose anyone is around who can speak with authority about how they did it in 1943, but does my theory hold water with folks who’ve thrown a scheme?

This site might give you some answers.

“Postal zones were introduced in larger areas in 1943. For instance, in New York, New York, postal zones generally progressed from the south of Manhattan north – zones 1 to 40. Furthermore, in The Bronx were New York zones 51 to 75.”

So it looks like NYC’s postal codes were assigned not just according to density but also following a grid system.

That’s not quite right, because the zone of Bayside, in Queens, was 64, and our zip code was 11364. There was one zone per post office, and of course one zipcode per post office when zip codes started. (And perhaps more - some buildings had their own zip codes, I believe.)

Instead of saying “NYC’s postal codes” I should have said “Manhattan’s postal codes.” The article I referenced mentions only Manhattan and the Bronx.

I cannot answer the OP, but one interesting fact is this. I lived in Champaign/Urbana IL, from 1964-68. When I arrived, they one zip code for Urbana and two for Champaign and they were 61801, 61802, and 61803. At some point in that four years, they changed to one zip for the twin cities. I though it was 61801, but I see that now Urbana is 61802 and Champaign is 61820.

Too many zips make the sorting job harder and the delivery easier, so there is a tradeoff.

Well, that’s not really what I see when I look at a map of Manhattan ZIP codes. I see 01 assigned to the main office, and that low numbers are generally in Lower Manhattan, but not in any real geographical order. Higher numbers are uptown—same as if they went more or less in chronological order of branch establishment, as I’m theorizing for Chicago.