Post Codes were introduced in the UK in the nineteen-sixties, although parts of Fermanagh weren’t absorbed into the system until the eighties or later.
Rather than adopting a five- or six-digit numeric system like most countries (such as the American Zip Code), the Royal Mail idiosyncratically developed a letter-plus-number system based on postal districts.
And it’s a fucking shambles. There is no consistent format – although the commonest is two letters, two numbers, space, one number, two letters (call it AA99 9AA), there are numerous variations. London formats are either AA9A 9AA or A99 9AA. Oh, wait a minute, they can also be AA99 9AA. Or A9 9AA. Other large cities — Glasgow, Liverpool, Birmingham etc. — generally have the A9 9AA form, but there’s no way to tell a priori if a city is “big” enough to be coded that way.
It’s impossible to design software that can validate a post code, except by exhaustively testing it against each known type, and Royal Mail introduces new ones from time-to-time, so you have to keep up. One programming website facetiously suggests that if you’re writing code to handle addresses, the only way to determine if a post code is correct is to have the software print and post a letter to it, and see if it arrives.
Royal Mail devoted a lot of advertising to educate the public in the post code scheme, and it’s become part of the fabric of life. Most of us add the post code when we address a letter.
Only one major organisation doesn’t use post codes: Royal Mail. The system is so unworkable that they had to develop a new, 5-digit numeric system called Mailsort for delivering mail. Just like American Zip Codes.