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These short rows of bars sit under an address and tell a sorting machine where to send the letter. The camera app does not scan them. The Zebra app does not scan them.

• Royal Mail 4-State Customer Code (RM4SCC)

  RM4SCC turns a UK postcode plus delivery point suffix into a row of four-state bars. Each character uses four bars, two of them long, and a final check character catches reading errors.

• Royal Mail Mailmark 4-state barcode

  Mailmark is Royal Mail's barcode system for business mail. Its four-state version comes in two lengths, type C and type L, and lets each letter be identified and tracked through sorting.

• KIX code (PostNL)

  KIX, short for Klant Index, is the Dutch version of the Royal Mail four-state code. It encodes the postcode and house number below the address, but leaves out the start, stop and check symbols.

• Australia Post 4-State Customer Barcode

  Australia Post's customer barcode encodes a Delivery Point Identifier in four-state bars. Longer variants add room for customer information, and Reed-Solomon error correction keeps it readable on sorting machines.

• Japan Post Customer Barcode

  Japanese business mail often carries a short row of four-state bars under the address. It encodes the seven-digit postal code plus a block and building number, ending with a check character.