what is the last 4 digits of a zip code called
ZIP Code In 1983, the U.S. Postal Service introduced an expanded ZIP Code system that it called ZIP+4, often called "plus-four codes", "add-on codes", or "add-ons". A ZIP+4 Code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail, a post office box, or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. But initial attempts to promote universal use of the new format met with public resistance and today the plus-four code is not required.[5] In general, mail is read by a multiline optical character reader (MLOCR) that almost instantly determines the correct ZIP+4 Code from the address—along with the even more specific delivery point—and sprays an Intelligent Mail barcode (IM) on the face of the mail piece that corresponds to 11 digits—nine for the ZIP+4 Code and two for the delivery point.
Seven-digit dialing The initial 86 area codes were assigned in 1947 as routing codes for operator calls; the first cross-country Bell System direct distance dial call was made in 1951. The system was based on fixed-length numbers; a direct-dial long distance call consisted of a three-digit area code and a seven-digit local number. Numbers in 2L+4N cities (such as Montréal and Toronto) were systematically lengthened to seven digits in the 1950's, a few exchanges at a time, so that all local numbers were seven digits when direct distance dialling finally came to town.