Non-mandatory ZIP codes were introduced to the United States in 1963. Robert Moon, a postal inspector with the United States Postal Service, proposed the ZIP code system as early as 1944.
The United States Postal Service began using two-digit area codes in 1943 in order to define postal zones in larger cities. Beginning in 1967, the ZIP code became progressively mandatory. The word "ZIP" comes from the acronym for "Zone Improvement Plan." The original ZIP code was composed of 5 digits. In 1983, the ZIP + 4 was introduced in order to make delivery to densely populated areas more efficient.