Nine-digit zip codes can be found on the U.S. Postal Service's website. Under the "quick tools" is an option to find a zip code. It prompts the user to enter an address and then locates the USPS ZIP+4 code.
"Zip" stands for zone improvement plan and was implemented in the United States in 1963 to organize the postal system and make it more effective. The first three digits in the original five-digit zip code signify the sectional center facility, which is a central processing facility. The last two digits represent the specific post office of the address. The last four digits were added in the 1980s and describe a more-specific location within the zip code.