RM4SCC

Royal Mail 4 State Customer Code (commoly abbreviated to RM4SCC) is a two-dimensional barcode that was develooped by Royal Mail in England. It is used for marking the postcodes of letters and parcels in the UK. The system, or one of its variants is also used in other countries. The printed code is optimized for being read at a high speed.
The barcode consists of a number of vertical bars. Each bar has one of four possible states. Four bars are necessary to code one of 36 possible characters. Two of the four bars point upwards, the other two point downwards. RM4SCC uses start and stop symbols. The RM4SCC barcode includes a checksum, which is also encoded as a character. 
There are other barcode systems, which are similar. PostNL, (formerly known as TNT Post), uses a system called KIX which stands for Klant index (Customer index). This system does not use the start and stop characters, and does not include a checksum.
Universal Postal Union has defined a standard how the system should be used.
Note that the code is read and printed at high speed, at which it is usually not possible to print straight lines; for this reason, the lines being "bent" is a normal state.