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The Zip+4 Postal Code (POSTNET Bar Code) is used by the U.S. Postal Service (USPS) for high-speed mail sorting.

POSTNET bar codes are placed on envelopes or postcards in one of two positions: on the lower right below the address or above the address. Use of this bar code on United States mail can save from 8% to 20% on postage costs. Contact your local post office for more information on correct placement of the bar code and the discount rate schedule.

The POSTNET bar code is made up of evenly spaced tall and short bars. There are 5, 9 or 11 digits plus a correction digit encoded into every POSTNET. The correction digit is used by a post office bar code reader to identify reading errors. Each numeric digit is represented by five bars. Two of these are tall and the other three are short. The tall bar represents a binary one (1) and the short bar represents a binary zero (0).

The nine-digit POSTNET (the most commonly used) consists of 50 bars representing the digits of the Zip+4 code and the correction digit, plus a framing
bar at each end of the string for a total of 52 bars. The five-digit type of code consists of 32 bars, and the 11-digit code has 62 bars.

Note: The 11-digit code is known as the delivery point bar code (DPBC). A DPBC is formed by adding 10 bars to the Zip+4 code to represent the last two characters of the primary street address or the last two characters of the post office box number.