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Explanation of US Zip Codes, or Why United States Zones Are the Way They Are!

Updated on February 4, 2015

ZIP Code Classifications

Upon seeing this request, I visited the USPS website. The information that I found about ZIP codes was quite interesting. For those of you that don't know, ZIP stands for Zone Improvement Plan and was put into action in 1963 by Postmaster General John A. Gronouski. On July 1, 1963, the Postmaster announced that the Zone Improvement Plan code would begin.

The digits mean specific things and here is what they mean:

The First Five Digits

  • First Digit: A broad geographic location.
  • Second and Third Digits: Pinpoints a more specific area.
  • Fourth and Fifth Digits: Designates Post Offices and/or Postal Zones

In 1983 an expansion was needed and became known as ZIP+4. This meant better and faster deliverability for our mail. The 4-digit add-on identifies the following:

  • City Blocks
  • Office Buildings
  • High Volume Mail Receivers
  • ... AND MORE!

The ZIP (Zone Improvement Plan) Code is vital to the USPS (United States Postal Service. If you have any further questions about the ZIP Code, please feel free to visit their website. For your convenience a link under the USPS logo above.

For more information about Zip Codes, please visit There is a link near the top of the page for them.
For more information about Zip Codes, please visit There is a link near the top of the page for them.


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    • profile image

      postal code 

      8 years ago

      nice its help us for my site

    • hamil331 profile image


      9 years ago

      I never knew there was this type of info in a zip code...thanks

    • ProCW profile imageAUTHOR


      10 years ago from South Carolina

      Thanks. Feel free to share this article with teachers and students!


    • SweetiePie profile image


      10 years ago from Southern California, USA

      This would be a good article for kids that are learning how to mail letters and about the US Postal service. I better go mixx it for you.

    • NateRider profile image


      10 years ago from Missouri

      I never understood the zip code system until today. I thought it was just a random number draw for counties in a state region. Now, I know otherwise. Good Hub

    • Kathleen in PA profile image

      Kathleen in PA 

      10 years ago

      Thanks, everyone!  I had 'fun' figuring it out - really!  Numbers have always interested me.

      Violette, you must have had fun w/the DB! I must "assume" that the odd/even rule is true, then, instead of N/S & E/W, due to pgmg experience.

      I recently created one in ACCESS for my high school class (won't tell you which year, ha ha). Didn't have opportunity to work on Data Bases on Mainframe through. - although everything ELSE!

      I haven't looked into the Area Codes yet...there are new ones cropping up all the time, and some have been split off due to overload/population.  Probably 'arbitrary', ha ha. Mine is 717, next-door neighbor's is 975 - Go Figure!


                               Go to your local Post Office, and TRY to get an answer on the ZIP CODE 'rule'!  Mine just said "We don't use the +4, and he never heard of the added two digits!!!                                                                     

    • An Again profile image

      An Again 

      10 years ago from Boston

      You know, I'd always wondered, but never thought to look it up myself. Thanks!

    • glassvisage profile image


      10 years ago from Northern California

      What a great idea for a hub!

    • glycodoc profile image


      10 years ago

      Thanks for taking the time to research this - interesting reading. Well done!

    • dutch84 profile image


      10 years ago

      You rock! I have wondered about this.

      What about area codes. Do you know how those are chosen?

    • stylezink profile image


      10 years ago from Atlanta, GA.

      Wow, never knew any of this before. I'm not sure what I'll do with it though; maybe save it for when I go on jeopardy, lol!

      Overall great hub and very informing.

    • SweetiePie profile image


      10 years ago from Southern California, USA

      Had never looked into this topic, but thanks for sharing this info. I answered a request about where to buy stamps, so our hubs have something in common :).

    • Violette DeSantis profile image

      Violette DeSantis 

      10 years ago from Broomall, PA

      I used to build matchkeys to link households in relational databases and yes the zip, zip+4 and first two digits of the house number are unique, unless we are talking multi-family dwellings. Can we talk MSA and FIPS codes now? LOL

    • Kathleen in PA profile image

      Kathleen in PA 

      10 years ago

      No problem! I was 'around' when they started, and I can't 'let go' of something that 'should' make sense, ha ha. The first part of the ZIP code came from a classmate of mine, whose father was Post Master in our hometown, and actually made the suggestion of using numbers. Don't know if it was HIS suggestion that got the ball rolling or not, but probable. One **correction** I made: not even/odd, but East/West or North/South side of the street, for last two digits of the +4. I certainly couldn't find any explanation on the USPS site!!

    • ProCW profile imageAUTHOR


      10 years ago from South Carolina

      Hey Kathleen. Thanks for more information on the organization of US Zip Codes! We appreciate it very much!

    • Kathleen in PA profile image

      Kathleen in PA 

      10 years ago

      The country is divided into 9 REGIONS, going from east to west, numbered 0 - 9.    Within each region are GEOGRAPHICAL AREAS, similarly numbered 0-9.  The third number represents the SECTIONAL POST OFFICE within that geographic area.

      Then the last two digits are for EACH post office, and are assigned ALPHABETICALLY, by city name.

      The + 4-digit number seems to indicate: pos 1-2:  Carrier Route.  Pos 3-4, East/West or North/South side of the street. It can also indicate a Post Office Box number.

      There is also an additional +2 numbers, that I 'think' are the last two positions of the house number.

      Ideally, these 12 numbers alone (without address) would get a parcel to the correct house, assuming the post office uses all 12 numbers.

      Additional comment:  There is to be NO COMMA between the city and state, on addresses:  100 Main St.  AnyCity   NY  nnnnn-nnnn-nn

    • mroconnell profile image


      10 years ago from France

      Thanks for the research. Maybe you should include a link to the website you talk about.


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