Introduced in 1983, the ZIP+4 code added a hyphen and four digits to the existing five-digit ZIP Code. The first five numbers continued to identify an area of the country and delivery office to which mail is directed. The sixth and seventh numbers denote a delivery sector, which may be several blocks, a group of streets, a group of post office boxes, several office buildings, a single high-rise office building, a large apartment building, or a small geographic area. The last two numbers denote a delivery segment, which might be one floor of an office building, one side of a street between intersecting streets, specific departments in a firm, or a group of post office boxes.
On October 1, 1983, the Governors of the Postal Service approved price incentives for First-Class Mail bearing the ZIP+4 code.
By the end of 1984, 252 OCRs were installed in 118 major mail processing centers across the country and were processing 24,000 pieces of mail per hour (an average productivity rate of 6,200 pieces per work hour) -- a substantial increase compared to the 1,750 pieces per work hour processed by MPLSMs.
