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Writing the wrong ZIP Code with the right address usually causes the letter to be delivered about a day later than if the ZIPCode were correct. As long as the mailing address is correct, the letter will be delivered.

A valid postal code depends on where you live. The specific order of the numbers of a ZIP code are important and can be used to help identify what state an individual lives in and where their mail is sorted. The first three digits of a ZIP code correspond to the central mail processing facility used to sort the mail.

So what do the extra numbers mean? These last 4 digits represent specific delivery routes within delivery areas. This extra detail means an even more precise matching to a more granular level. It could represent a handful of houses on one side of a street, or even a single building that receives a high volume of mail.

This is because ZIP code boundaries are based on mail routes and delivery points rather than geographic factors. The U.S. Post Office Department, predecessor to USPS, developed a zoning address system in 1943 as a way to make sorting and delivery of mail easier.

Data scientists should know that ZIP codes do not always fall within state boundaries (or even of the borders of states within a zone). There are over 100 cases where ZIP codes cross state lines.

It is preferred that all City, State, and ZIP Code information be on a single line. If that is not possible, the ZIP Code, including the ZIP+4 Code, may be placed below the city/state information. However, when compressing line #11, do not remove the hyphen between the ZIP Code and the ZIP+4 add-on code.

Not Always. When looking at postal codes around the world, there are cases where two different cities are served by the same postal code. For example, the zip code 94608 in California is used for both Emeryville and parts of Oakland.