system of zone coding introduced by the U.S. Post Office Department (now the U.S. Postal Service) in 1963 to facilitate the sorting and delivery of mail. After an extensive publicity campaign, the department finally succeeded in eliciting from the public a widespread acceptance of the ZIP code. Users of the mails were requested to include in all addresses a five-number code, of which the first three digits identified the section of the country to which the item was destined and the last two digits the specific post office or zone of the addressee. The primary purpose of the zone coding system was to fully exploit the capabilities of electronic reading and sorting equipment.

The U.S. Postal Service introduced a nine-digit ZIP Code in 1983. The new code, composed of the original five digits plus a hyphen and four additional numbers, was designed to speed up automated sorting operations. The first two of the four extra digits specify a particular sector, such as a group of streets or cluster of large buildings. The last two digits of the expanded code represent an even smaller area called a segment, which may consist of one side of a city block, a single floor in a large building, or a group of post office boxes.