The ZIP code is a system of postal codes used by the United States Postal Service (USPS) since 1963 and is often called ZIP. The letters ZIP are an acronym for Zone Improvement Plan and must be written in capital letters. The term ZIP was chosen to suggest that postal mail travels more efficiently, and therefore faster, when senders use the code. It was chosen to suggest that mail travels more efficiently, and therefore faster. , when senders use the postal address code. The basic format is made up of five numeric digits. In 1983 the ZIP + 4 zip code system was introduced in the US, adding four digits to the end of the old five-digit codes. The four added digits can help determine a more precise location. The term "ZIP Code" was initially registered as a service mark (a type of trademark) by the United States Postal Service, but the registration has since expired.