The POSTNET (Postal Numeric Encoding Technique) symbology was developed in the United States for the automatic sorting of mails. POSTNET is only used for the mailing address marking. POSTNET is not a real barcode (as a barcode is coded by bars of variable width, while a POSTNET is encoded by bars of variable height). It is a numeric symbol that uses five bars (two long and three short) and four spaces for each coded character. The length of bars, and the spaces between them, is constant throughout the code. A single bar is used for the start and stop characters and the symbol generally include a check character.
A Postnet barcode has a starting frame bar, followed by 5,9, or 11 data characters, followed by a check digit, and a stop frame bar.