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Number only codes had proved too difficult to remember and have a small number of permutations (10 per digit), while letters have 26 permutations per letter. Combining letters and digits give you a much greater range.

Codes had to be short, memorable and sort to street level. It was decided that each code would begin with the initial letters of the town name (the outward code), for example, NOR = Norwich. These have since been revised and include a number (e.g. NR1, LS4, etc)

The outgoing code is the most memorable, so if there are errors in the second half of the code (the inward code), at least it would still get to the right area.

The inward code consists of 1 digit and 2 letters, corresponding to the street - this gives you a very large number of possibilities (10x26x26)

The number is placed in the middle because it stands out and can assist recall, acting as cue for the final 2 letters where errors are most likely to occur.

You can send a letter by using just the house number and the postcode, but Royal Mail don't like that.

Although originally designed to speed up sorting, postcodes have become a vital part of our everyday lives, from Sat Navs to ordering takeaway.