RM4SCC
Royal Mail's four-state barcode, where every bar carries a top tick, a bottom tick, both or neither — a picket fence encoding the postcode.
The name stands for "Royal Mail 4-State Customer Code". Unlike a shop's width-modulated codes, each bar carries information in its shape: it can have an ascender, a descender, both or neither — four states per bar. Two bars encode one character, and the whole row ends in a checksum digit.
Royal Mail introduced it in the 1990s for machine sorting of post marked with a postcode, sometimes called RM4SCC or the "Customer Barcode". The four-state principle packs more per millimetre than the American POSTNET code's pure height modulation, and the same family of fence codes appeared at several postal services. It is read by the same sort of barcode scanner as other linear codes, if with special decoding.