The USPS Intelligent Mail (4-State Customer Code) encodes a 20, 25, 29 or 31-digit string of mailpiece data into 65 vertical bars. The density of the code is 20 to 24 bars per 25.4mm. All bars must be equally spaced apart.
Each bar can be in one of four "states", thus the name of the symbology. The four states are "Tracker", "Ascender", "Descender", and "Full Bar" and are illustrated in the diagram below.
The USPS Intelligent Mail (4-State Customer Code) can be used on letter-size and flat-size mail. In order to correctly position a USPS Intelligent Mail (4-State Customer Code), it must be placed in the following locations on the envelope.
Above or below the address within the address block (No POSTNET barcode used).
Above the address within the address block (The POSTNET barcode positioned below the address within the address block).
Below the address within the address block (The POSTNET barcode positioned above the address within the address block).
Above the address within the address block (The POSTNET barcode positioned either below the address within the address block or in the barcode clear zone).
The Character Set