<P> In 1967, these became mandatory for second and third - class bulk mailers, and the system was soon adopted generally . The United States Post Office used a cartoon character which it called Mr. ZIP to promote the use of the ZIP Code . He was often depicted with a legend such as "USE ZIP CODE" in the selvage of panes of postage stamps or on the covers of booklet panes of stamps . </P> <P> In 1971 Elmira (NY) Star Gazette reporter Dick Baumbach found out the White House was not using a zip code on its envelopes . Herb Klein, special assistant to President Nixon, responded by saying the next printing of envelopes would include the zip code . That did occur . </P> <P> 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 . However, initial attempts to promote universal use of the new format met with public resistance and today the plus - four code is not required . 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 . </P> <P> For Post Office Boxes, the general (but not invariable) rule is that each box has its own ZIP + 4 code . The add - on code is often one of the following: the last four digits of the box number (e.g. PO Box 107050, Albany, NY 12201 - 7050), zero plus the last three digits of the box number (e.g., PO Box 17727, Eagle River, AK 99577 - 0727), or, if the box number consists of fewer than four digits, enough zeros are attached to the front of the box number to produce a four - digit number (e.g., PO Box 77, Juneau, AK 99750 - 0077). However, there is no uniform rule, so the ZIP + 4 Code must be looked up individually for each box . </P>

What are the extra four digits at the end of a zip code