At the risk of telling you what you already know, ZIP codes were, as the name says, part of a "zone improvement plan." Prior to ZIP codes, we used zone numbers, which were two digits and went between the city and the state in a mailing address (e.g., "Brooklyn, 28, NY"). The ZIP code added three numbers that helped locate the piece of mail by state, region, etc. What had been the zone number became the last two digits of the ZIP code ("Brooklyn, NY 11228")
At the risk of telling you what you already know, ZIP codes were, as the name says, part of a "zone improvement plan." Prior to ZIP codes, we used zone numbers, which were two digits and went between the city and the state in a mailing address (e.g., "Brooklyn, 28, NY"). The ZIP code added three numbers that helped locate the piece of mail by state, region, etc. What had been the zone number became the last two digits of the ZIP code ("Brooklyn, NY 11228")