Royal Mail 4-State Customer Code is used for the Royal Mail Cleanmail service. It enables UK postcodes as well as Delivery Point Suffixes (DPSs) to be easily read by a machine at high speed.
Each character is made up of 4 bars, 2 of which extend upward, and 2 of which extend downward. The combination of the top and bottom halves gives 36 possible symbols: 10 digits and 26 letters.
As the example right shows, the barcode consists of a start character, the postcode, the Delivery Point Suffix (DPS), a checksum character, and a stop character. The DPS is a two-character code ranging from 1A to 9T, with codes 9U to 9Z being accepted as default codes when no DPS has been allocated