When you write a U.S. address on an envelope, the last five numbers are the ZIP code. Don't forget the ZIP code on that thank-you note to your grandparents!
Everyone in the U.S. who has a home has a ZIP code. While most of these are five digits, many ZIP codes now include an additional four numbers that help mail get delivered accurately and quickly. Modern ZIP codes were introduced in 1963, along with standardized two-letter abbreviations for each state. While using the ZIP code (short for "zone improvement plan") wasn't mandatory at first, people soon realized that using them really did help the mail zip along more quickly.