Grammar Writing Zip Codes |
The Blue Book of Grammar and Punctuation

Writing Zip Codes

We at times receive questions concerning the proper way to address outbound mail and, in particular, use zip codes. Because your goal and ours is to achieve precision in writing, we’ll take a closer look at zip codes and how you can treat them in your correspondence.

What Is a ZIP Code?

Zip codes today are five-digit numbers at the end of U.S. mailing addresses to help ensure more-efficient delivery in the American postal system. Without them, mail can be delayed because of extra manual handling.

According to the Smithsonian National Postal Museum, what we now know as zip codes were introduced in 1943 as a result of World War II. In addition to more mail being sent during the war, the postal service was hiring many new workers to replace those who had left and were leaving for battle. To help the large number of inexperienced employees, the postal service created a new system of numerical postal zones that would simplify the sorting and sending of mail (zip stands for zone improvement plan).

In the beginning, the numbered postal zones were used only in 124 of America’s largest urban areas. An original address might have looked like this, with the number identifying the city:

Gary Grammar
1234 Apostrophe Way

Indianapolis 24, Indiana

By the early 1960s, the American post needed a more detailed and organized system, so non-mandatory five-digit zip codes were introduced along with two-letter state abbreviations (e.g., Chicago, IL 60601).

In 1983, the zip + 4 format we now use (e.g., 60601-1635) was introduced to support delivery to densely populated areas. The first three digits before the hyphen identify the sectional center or large city. The second two classify the post office facility or delivery area. After the hyphen, the first two digits indicate the sector or group of blocks, and the last two identify the segment or one side of a street.

Writing Zip Code: A Matter of Style

Sources differ concerning how to write the phrase for the U.S. mail’s identifying numerical system. The U.S. Postal Service uses ZIP Code. It also includes a trademark symbol (ZIP Code) even though available information suggests the mark expired in 1997.

The Associated Press Stylebook advises using ZIP code. The Chicago Manual of Style favors zip code, which is the style that observes.

You would be correct to use any one of these choices unless directed otherwise by an established preference or style guideline.

Writing Zip Codes: Content vs. Mail

If you are writing an address with a zip code in content, style guides often agree on including a comma between the city and state (e.g., Chicago, IL) in the city-state-zip code line.

AP would use the two-letter state code in both content and outbound mail:

Chicago, IL 60601 Tulsa, OK 74120
Santa Clara, CA 95053 St. Petersburg, FL 33731

CMOS would spell out the state unless it appears in lists, tabular matter, notes, bibliographies, indexes, or outbound mail (in which case it would follow AP style):

Chicago, Illinois 60601 Tulsa, Oklahoma 74120
Santa Clara, California 95053 St. Petersburg, Florida 33731

When we are writing zip codes for the U.S. mail, the postal service prefers that punctuation not be used. It also prefers that lettering be in all CAPS; the address be printed on the center of the envelope with a flush left margin; and the + 4 number be included.


To further explore written format for the U.S. mail, you can review our related topics:

Writing Addresses
Commas to Separate a City and State

If the article or the existing discussions do not address a thought or question you have on the subject, please use the "Comment" box at the bottom of this page.

12 responses to “Writing Zip Codes”

  1. Greg says:

    “…(zip stands for zone improvement plan).”

    Then why is “zip” not capitalized?

    PS. Love your USPS-standard mailing address.

    • says:

      As the post states, The Chicago Manual of Style favors zip code, which is the style that observes.

  2. Kristine Anderson says:

    Why do you use the plural “indexes” and not “indices” in this article?

    Thank you.

    • says:

      Both Merriam-Webster and American Heritage Dictionary allow for either spelling. Our site represents American English rules and guidelines. Indexes is a more common spelling in American English, and indices might often be used among those who prefer a more formal or prescriptivist spelling.

  3. Prof. Valerie Whitney says:

    AP Style has changed the rule when it comes to state names.The names of all 50 state should be spelled out when used in the body of a story, regardless of whether it is alone or with the city.

    • says:

      AP seems to use the state’s abbreviation when followed by a zip code.
      The following is AP’s entry on zip codes:

      Run the five digits together without a comma, and do not put a comma between the state name and the ZIP code: New York, NY 10020.

  4. Roy Warner says:

    My beloved uncle was a young United States Post Office employee in the Bowling Street station in NY; the US Post Office was then a cabinet department. Like other young men in the Post Office, he was drafted into the US Army in 1940 (the first peacetime draft) to help create the APO – the Army Post Office, as part of the military buildup for what became the inevitable entry of the US into WWII.
    Despite what might be in the US Archives, the reference to a “ZIP” code wasn’t made until the early 60s, which was based upon a “zone improvement plan” designed by a US Post Office employee; I read his obituary in the NY Times many years ago. The first improvement during the War simply provided a means by which to make the post office station readily identifiable. Thus, when I grew up in the 50s in Flushing, NY, my code was Flushing 65, NY, the “65” being the Fresh Meadows Post Office Station, which was within the broader Flushing area. The changeover to what we call the ZIP Code converted Flushing into 113, with 65 being Fresh Meadows.

  5. Ken says:

    I’m surprised it’s acceptable to stylize it as anything other than “ZIP” because it’s (1) an acronym and (2) it has nothing to do with the word “zip.”

  6. Kay Gold says:

    As a person who sends mail regularly to the USA (from the UK), I think I address mail mostly as the AP would have it. Next time though, I’ll capitalise and miss out the commas. However, my question is regarding the country name.

    Where does the ZIP code go then?

    TOWN FULL-STATE-NAME.(here? Option 1)
    USA ZIPCODE* (or here option 2)

    *only 5 digits; I don’t know the other 4.

    • says:

      To send mail to the United States from another country, it is our understanding that you write “USA” after the zip code on a separate line.

  7. Damon says:

    Is there one space after the state and before the zip code or two? I was raised to put two spaces and the post office still recommends this. “Leave one space between the city and state and two spaces between the state and ZIP Code™.”

    • says:

      It seems that USPS has conflicting information on the website. We did find your quotation in Appendix B. However, according to USPS Publication 28 titled “Postal Addressing Standards,” the examples of addressing standards for United States mail below are current with one space. If you are concerned about the discrepancy, you may wish to contact USPS.

      1500 E MAIN AVE STE 201
      SPRINGFIELD, VA 22162-1010

      JOHN DOE
      123 MAIN ST
      ANYTOWN US 12345-9998

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