The United States Postal Service as we know it today had its beginnings at the birth of the country itself. The mail service has the unique distinction of being one of a few governmental agencies explicitly authorized by the United States Constitution, under the so-called "Postal Clause." Although postal services had been active in the colonies before independence, the service flourished following the establishment of the United States Post Office in 1775. Over the next few decades, the postal service was extended westward, as Congress authorized the construction and establishment of postal routes across the country.
The 19th century saw a massive surge in the development of postal services in the United States. The population of the country increased tenfold in the period from 1790 to 1860, and the postal offices and employee numbers reflected this growth – the number of post offices increased from 75 to over 28,000 in this period. New methods of mail delivery were pioneered, such as steamboats and railroad, but quicker service to the Pacific Coast was needed. This led to the development of the iconic Pony Express – a system using hardy horses, good riders, and relay stations 10 – 15 miles apart to ensure fresh mounts for the long journeys. Although the system only ran for 18 months, it remains an iconic symbol of American postal history. At a time when air travel was just an experiment, the Post Office was instrumental in investigating the possibilities of mail delivery via air. Their efforts paid off, and by 1918, Congress approved a large sum to be devoted to providing airmail routes and the system has gone from strength to strength since.
In order to offset the loss of employees who became soldiers during World War II, the Post Office Department introduced the concept of a zoning address system in several large cities. Eventually this developed into the Zoning Improvement Plan (ZIP) code system, and by July 1963, every home in the country had been assigned a 5-digit code. By the end of the 1960s, the US Post Office was struggling financially and reform was desperately needed. In 1970, comprehensive legislation was introduced and by 1971, the US Post Office was transformed into the United States Postal Service. The Postal Reorganization Act changed the face of the organization, particularly relating to its labor relations, financials and transportation.
The USPS added four digits to the existing five-digit zip codes in order to increase efficiency and speed of mail handling and delivery in 1983.