Inaugurated in 1775, the United States Post Office has faced many challenges in trying to bring mail to far-flung areas and cities of often rapidly growing size. In 1943, it instituted rudimentary codes to addresses in large cities for the purpose of better organizing, sorting, and delivering mail. The development of suburbia after World War II only exacerbated the challenges of mail delivery, and the Post Office found a solution in the 1963 establishment of the Zone Improvement Plan, or ZIP code, a non-mandatory number added to the end of an address. This new number was grafted onto any numbers used previously, and the system was set out to cover the entire country in an organized fashion. This created encryptions with both general and specific destinations.
When a letter is mailed, it goes through a number of steps. First, the mail is carried to one of twenty-one Network Distribution Centers. Each of these are identified by three numbers, which match the first three digits of that center’s ZIP code. Twenty-one centers for the entire country makes for a very rough sorting; the purpose here is to divide the mail into groupings that can be further sorted later, getting the letter closer and closer to its recipient.
Sectional Center Facilities are the next stop for the letter, and there are a great many more of these; while Hawaii has only one for the entire state, Indiana has six, and California sixteen. These centers will now match the first three digits of the letter’s ZIP; the Grand Junction Sectional Facility gathers mail for all codes beginning with 814-816. Here, the mail is once again sorted, and then sent to Post Offices in individual areas.
To put this all together, let’s look at a letter sent to someone living in Fort Worth, Texas, who uses a ZIP code of 76112. Mailed from Portland, Oregon, the letter first goes to the local post office and then sent to a central processing facility. Here, it is again sorted and then sent to the Dallas NDC, or 753 – closer, but not yet matching. The mail is sorted a fourth time before going to the Forth Worth SCF, which serves 760 to 764 and 766 to 767. Finally, the letter goes to the post office serving the full ZIP code, where it’s then sorted by street and address. At last, the letter finds its recipient!
To be sure, it’s still possible to send a letter without a ZIP code; for that matter, there are still some communities where the postmaster can take a letter mailed to “Lydia Smith, Corning, CA” and make sure it arrives. However, letters that don’t follow the approved addressing format can’t be sorted by machine, which adds time to their delivery. Furthermore, someone will then have to take the time to look up and add the missing information, which further slows down that letter’s arrival date. The ZIP code is therefore a very useful tool for expediting mail delivery.