Why are UK postcodes so memorable?

Postcodes aren’t exactly interesting, at least that’s what I thought. I’d been reading about how cognitive psychologists from Cambridge University had been involved in the design of the UK postcode system in the late 1950s and happened to mention it briefly on Twitter, a kind of admission of my own nerdiness.

A couple of my followers replied suggesting I create a thread about what I’d learned. I suspected they were either joking or were just as nerdy as me. I duly obliged, hoping that someone might find it interesting.

Less than 24 hours later my thread was trending. People had started numerous discussions about how the UK system differs from those in other countries, how I’d neglected to include some of the more idiosyncratic elements of postcodes, and pointing out that London postcodes are different to the rest of the country. I also received some lovely messages from postal workers highlighting how these curious combinations of letters and numbers shape their working day. So what is it about UK postcodes that people appear to find so interesting?

My main interests lie in the psychology of learning; with the practical application of psychological research. Often, however, psychology seems to be somewhat detached from the real world. This perception is perhaps more obvious when it comes to cognitive psychology (the study of mental processes). Cognitive psychologists seem to spend a large amount of time getting volunteers to recall lists of names, numbers and words that often appear to have no relationship to what people are actually doing with their memories. Appearance can be deceptive, however, and cognitive psychology has provided many answers and potential solutions to a range of factors, from the cognitive mechanisms involved in dyslexia and dyscalculia to designing more effective ways the police can interview witnesses. Back in the 1950s, the Post Office even asked cognitive psychologists to help them design a memorable postcode system.

UK Postcodes: The basics.

The city of Leeds & surrounding area with postcode districts

Initially, postcodes were designed to speed up sorting following the mechanisation of the postal system in 1959, but they did exist in some areas before this, such as London where the codes correspond to compass points and postal districts (e.g. N1, E17, NW1). However, today they are used for much more than sending letters. A Sat Nav, for example, is much easier to use if you know the postcode of your destination as is ordering that takeaway during lockdown. I recently moved to Leeds from a town in North Yorkshire and, although I know Leeds quite well, I wasn’t at all sure where I wanted to live. I asked friends and family and they suggested areas I might like (or might like to avoid). Often, however, they would reply with ‘LS6 is okay if you don’t mind all the students but LS4 is smaller and a little quieter’ or ‘LS9 and LS2 are close to the city centre.’ I then searched properties online using the outgoing postcode. Of course it’s not always so simple. Skipton, a small market town in the county of North Yorkshire, has a Bradford (BD) outgoing code (Bradford is in West Yorkshire and around 18 miles from Skipton).

London postcodes follow a pattern based on compass points

The Psychology

The codes they were asked to develop had to memorable and sort to street level; they also had to ensure a maximum number permutations. Research carried out in the 1950s had already found that short-term memory had a capacity of between 5 and 9 pieces if information (letters, numbers, words, etc). This meant that the codes couldn’t have more than 9 letters or digits because the average person would be unable to hold them all in short-term memory. In other words, if I were to recite a list of numbers to you and ask you to recite them back to me, you should be able to cope with 7 or 8 but not 10 or 11 (technically, this is known as the immediate digit span).

Research also concluded that codes with only numbers proved difficult to remember; they also have a smaller range of permutations than letters (10 versus 26). A mixture of numbers and letters, on the other hand, would produce a significantly higher number of permutations. But you then have to decide the order of the letter-number combinations.

It was decided that each code would begin with the initial letters of the town name, so Norwich (where the first postcodes were trialled in 1959) would initially become NOR, York would become YOR (they would later be adapted to NR and YO followed by a number). This part is the easiest to remember because it includes elements of the town or city’s name and because it’s the part less prone to error, even if the rest of the code is wrong it’s still going to end up in the right area.

The second part of the code (the inward code) is the most difficult to remember and corresponds to the street. By using a digit and 2 letters it can produce a huge number of possibilities (10 x 26 x 26) but the Post Office were unsure of where to place the number. Conrad opted to place it at the beginning of the inward code because it stood out and acted as a mental nudge for the final two letters (this is were most errors occur, often through reversal; BA becomes AB, for example). The gap between the two codes creates a natural rhythm in the same way as telephone numbers.


Thanks to cognitive psychology, the United Kingdom has one of the most memorable postcode systems in the world, if not the most memorable. It may have been intended to to speed up sorting but with the advent of the internet and, most recently, the smartphone, it’s become an increasing useful product of early cognitive psychology.

Chartered Psychologist. Behaviour Change. Becoming Buoyant — out now https://www.amazon.co.uk/dp/0367441624