This poster from the 1970s encouraged people to use their postcodes
The Royal Mail is to launch a campaign to celebrate the 50th anniversary of the postcode.
It will stress the importance of using the letters and numbers that make up postcodes on mailed items.
Almost a fifth of non-business letters, cards and packets are sent without a full or accurate postcode.
Giles Finnemore, of Royal Mail, said the postcode was still important "to help Royal Mail sort and deliver mail quickly and efficiently".
There are now more than 1.7 million postcodes across the UK, covering 27 million addresses.
Postcoded letters are read by Royal Mail's machinery and sorted 30 times faster than those dealt with by hand.
The first postcodes were introduced in Norwich in 1959, using the letters NOR, before being rolled out across the UK in the 1960s.
Postcodes are used widely for non-postal services, such as online shopping or satellite navigation systems.
The first steps towards the modern day postcode were taken in 1857 when Sir Rowland Hill, inventor of the postage stamp, introduced a scheme to accelerate mail delivery.
This divided the capital into 10 separate postal districts - N, S, E, W, NE, NW, SE, SW, EC and WC. The S and NE codes have since been reassigned to the Sheffield and Newcastle areas respectively.
The public were asked to add these district letters to the bottom of written addresses, to help speed up delivery.
POSTCODE FACTS
The UK has 1.7 million postcodes
The Royal Mail online postcode checker receives about 4.5 million hits a month
Each postcode covers an average of 15 addresses
Buckingham Palace's postcode is SW1A 1AA
Wembley Stadium's is HA9 0XX
Father Christmas has his own postcode - SAN TA1