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A '''ZIP code''' is a five-[[digit]] [[numeric]] [[code]] used by the [[United States Postal Service]] in sorting [[mail]]. The first digit indicates a group of [[state]]s; the second and third, a region of a state or a large [[city]]; and the fourth and fifth, a small [[town]], a delivery area in a larger town or city, a particular postal facility, or a particular large mail recipient.  Extensions of the ZIP code include the nine-digit ZIP+4 code and the 11-digit ABC (Advanced Bar Coding) code.

A '''ZIP code''' is a five-[[digit]] [[numeric]] [[code]] used by the [[United States Postal Service]] in sorting [[mail]]. The first digit indicates a group of [[state]]s; the second and third, a region of a state or a large [[city]]; and the fourth and fifth, a small [[town]], a delivery area in a larger town or city, a particular postal facility, or a particular large mail recipient.  Extensions of the ZIP code include the nine-digit ZIP+4 code and the 11-digit ABC (Advanced Bar Coding) code.

Zip codes are encoded into barcodes using a redundant binary code and stamped on letters.  A "1" (tall line) starts and ends the zip code, and immediately before the final "1" comes a [[checksum]] digit which, when added to all five digits of the zip code, adds to a multiple of 10.  The code scheme is chosen so that each digit's binary equivalent has exactly two "1"'s.  Digits are encoded as follows:

ZIP codes are encoded into barcodes, using a redundant binary code with tall lines to represent the bit "1" and short lines to represent the bit "0," and printed on mail.  A "1" (tall line) starts and ends the ZIP code, and immediately before the final "1" comes a [[checksum]] digit which, when added to all of the digits of the ZIP code, adds to a multiple of 10.  The code scheme is chosen so that each digit's binary equivalent has exactly two "1"'s.  Digits are encoded as follows:

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