Delivering the right letter to the right address isn't as easy as it might at first seem. In small towns and villages there is probably only one instance of each street name, but in larger towns and cities there may be several. Which 'London Road' should the letter be delivered to? There may also be villages with similar names at opposite ends of the country. Many countries around the world have developed postal codes which are added to the end of an address to help with this problem. France (where almost every village seems to name one 'rue' after Victor Hugo and another after Anatole France) and the United States use a collection of numbers, while the United Kingdom is one of the countries that use a combination of letters and numbers.

History of Postcodes

During the 1840s, the number of letters being sent in London was increasing rapidly. With many localities having similar street names, letters were often misdirected. In the 1850s a committee was instructed to find a way to stop the confusion. They originally planned to rename the streets, but many residents objected, so they decided instead to split the city into various sectors. The two central sectors were EC and WC (east and west central) and the outer ones were named N, NE, E, SE, S, SW, W and NW after the points of a compass. A scheme which involved people adding these letters to their addresses was implemented during 1857 and 1858. In 1866 Anthony Trollope, then a surveyor, suggested that NE be merged into E. Then S vanished two years later, after being split between SE and SW.

A number of major cities such as Liverpool, Manchester and Glasgow followed London's example by splitting up the mail delivery into geographical areas represented by numbers. In Liverpool, some people still refer to areas by these sectors, saying they live in 'Liverpool Eight' or 'Liverpool Nine'. By 1917 the London sectors had been split again into smaller divisions such as E1, E2 etc.

The first trial of a full six-digit postcode system was attempted in Norwich in 1959. Letters were meant to have the code NOR for the city and then three characters representing a street. The trial was not a great success, as fewer than half the letters sent had these codes on them. Lessons were learnt, and a new trial was started in Croydon in 1966. Integral to the trial was an automatic sorting machine to route the mail. To enable the sorting machine to be able to 'read' the postcodes, an operator keyed in the information which was then converted to blue phosphor dots - a machine-readable version of the postcode - and printed on the envelope. The postcode system was rolled out across the country, and ended with the re-numbering of Norwich in 1974.

In 1985 OCR (Optical Character Recognition) systems were introduced, which automatically read the postcodes and printed out the phosphor dots, instead of them being printed manually. Ten years later, the phosphor dots were replaced by a code based on pink dots.

Dissecting a Postcode

Let's use this example postcode PO1 3XA. First of all it is split into two parts, the Outward – PO1 and the Inward – 3XA. The Outward gets it to the correct delivery office; the Inward tells that office what to do with it. The Outward code can be anything from two to five characters long. The Inward has to be three characters long.

Postal Area

The first letter or two letters of the postcode dictate to which one of the 124 postal areas it should go. In the case of PO, this means Portsmouth is the main post town.

Postal District

The first one or two numbers of the postcode represent which delivery office the mail should go to. There are 2,400 postal districts in the UK. In the case of London, the postal district is sometimes a combination of numbers and letters as the numbers are still based on the 1917 districts and so have to be split apart to provide more capacity.

When the London districts were numbered, the one with the main sorting office for the postal area was numbered 1, then the smaller sorting offices were numbered in alphabetical order. This means that for E, E1 is where the main sorting office is and contains Wapping, Whitechapel, Shoreditch and Stepney; E2 is Bethnal Green; E3 is nearby Bow; but E4 is Chingford which is much further out. The boy band East 17, who had several hits in the 1990s, took their name from their local postcode in Walthamstow.

Some towns and cities have districts (like M60 in central Manchester) which do not exist geographically: they are assigned to some of the larger establishments in the centre for ease of distribution.

To return to our letter, PO1 means it goes to the first postal district in the Portsmouth area, which happens to be Portsmouth itself.

Postal Sector

The number in the Inward code (the second part of the postcode) directs the letter to one of more than 9,000 postal sectors. These help split up the mail for the individual post routes.

Our letter goes to the people dealing with sector 3.

Walk

The final two letters of the postcode narrow the address down to a group of buildings, perhaps a street. It is stated that there should be fewer than 80 buildings in a walk, though the average is more like 14. A walk was meant to be just one postman's route, but some walks are small enough to allow more than one to be included in a round.

Many companies get enough mail for them to have their own walk code. Some large companies have a couple of walk codes which means that the mail delivery companies pre-sort their mail for them.

The XA walk sees our letter delivered to the Royal Mail offices in Portsmouth.

The Postal Address

The address for each item of mail should consist of these elements (where applicable) in this order (an example of each has been provided):

• Addressee identification - ie: Mr AN Other
• Building - Anon House
• Dependent thoroughfare - Random Court
• Thoroughfare - False Name Road
• Double dependent locality - The Made Up Estate
• Dependent locality - Nosuchtown
• POST TOWN - Manchester
• County - Greater Manchester
• POSTCODE - M20 9TY

There are 1,500 post towns in the country. These are major towns and cities that are defined by the Post Office and are used for routing letters, especially when a postcode is not included. It is worth noting that post towns are not associated with individual postal areas and postal districts. Some post towns, like Manchester, can be the post town for various postal districts, while some postal districts can have more than one post town.

The great thing about postcodes is that it means people can reduce the amount of information on the address of any mail they are sending. If you have the postcode and post town correct, there is no need to include your county¹ in the address. In theory, a letter can reach its recipient with just the house number, street name, post town and postcode.

The Problem with Understanding Postcodes

Postcodes were introduced to help sort mail. Because they are often seen with the names of towns and counties on addresses, many people wrongly assume that postcodes are based on certain geographical or administrative boundaries.

People assume that certain postcodes mean that their properties belong in certain counties. Postcodes remain constant while county boundaries change all around them. Nowadays some postal areas can stretch across a couple of counties, in fact in Northern Ireland, the BT (Belfast) area covers the whole province.

People will claim that because they have a certain postal area and they can include a county in their address line, they belong in that county. As we've seen, postal areas are not based on county boundaries and it doesn't matter what county is included in the address line, the post office will ignore it.

London Postal Areas

London postal areas are probably the most misunderstood of the lot. When the original set of London postal areas were drawn up, they spread outside the boundaries of what was then London County. When Greater London was created in 1965, it encompassed suburbs in what had been other counties. The new additions retained their postcodes, meaning that all or part of the EN Enfield, KT Kingston-upon-Thames, HA Harrow, UB Uxbridge, TW Twickenham, SM Sutton, CR Croydon, TN Tonbridge, DA Dartford, BR Bromley, RM Romford and IG Ilford postal areas are within the boundaries of Greater London.

Because these suburbs don't have one of the EC, WC, N, NW, W, SW, SE, E postcodes, people will claim they do not have London postcodes and are still part of their original county.

The Postcode Lottery

Postcodes are much more important in everyday life than just as a method of routing mail. As they are quick to write, familiar and unique, postcodes have become beloved by statisticians and are sometimes used in the media to demonstrate inequalities throughout the nation.

A database exists that categorises residents of each postcode into one of 56 distinct groups based on income, ethnicity and employment. Social researchers have produced reports on how the average household in each of these groups lives. These reports include everything from car ownership and amount of television watched to how likely they are to buy frozen sausages. Marketing companies can use this data to target their campaigns, while homebuyers can use it to find out whether or not they want to move into the area.

People will pay a premium to live in one postal district rather than another. Insurance companies may also charge a much higher premium for one person compared with another a few streets away, just because of a client's postcode. The differences in prices, schools and doctors between one area and another is often referred to by the media as the postcode lottery. In Manchester for example, there is a premium on houses in the M20 (traditionally Withington and Didsbury) over nearby M19 (Burnage) and M14 (Fallowfield).

In-car GPS systems can use postcodes for finding addresses. This has good points and bad points. Since postcodes are unique, you know that you are heading to the right 'Chatham Grove'. However, since some postcodes can cover more than one street, drivers may be directed to the wrong road!

Some Notable Postcodes

• SAN TA1 - Father Christmas
• GIR OAA - Girobank
• RM1 1AA - Royal Mail Customer Service
• E20 - Walford (Eastenders)
• SW1A 1AA - Buckingham Palace
• SW1A 0AA - House of Commons
• SW1A 0PW - House of Lords
• SW1A 2AA - 10 Downing Street
• SW1A 2AB - 11 Downing Street
• W1A 1AA - BBC Broadcasting House

Various UK dependencies have their own postcodes. As these places are generally so small, the postcode covers the entire territory. Having a postcode not only makes the residents feel as if they belong to the UK (and stops mail for St Helena ending up in Merseyside), it also allows them to order goods online². Some of these dependencies are:

• ASCN 1ZZ - Ascension Island
• BIQQ 1ZZ - British Antarctic Territory
• BBND 1ZZ - British Indian Ocean Territory
• FIQQ 1ZZ - Falkland Islands
• PCRN 1ZZ - Pitcairn Islands
• STHL 1ZZ - Saint Helena
• SIQQ 1ZZ - South Georgia and the South Sandwich Islands

How I Learnt to Stop Worrying and Love the Postcode System

The postcode system is a remarkably simple solution to the vast problem of routing letters. It has become so familiar to the population that many people take it for granted and have little actual knowledge of how it works. While it is immensely helpful for the Post Office in locating addresses, without a vast knowledge of the postcode system it is of little use to the man on the street, except to perhaps point people to a general area. If you want to find where you are going, or which county a town is in, don't rely on a postcode; use a map.

And always remember to pass on your postcode!