Here in the UK today is one of the two dates in the calendar for new car registrations. For the 20th year in a row licence plates issued on 1 March (standard issue, not “cherished” or customized plates) reflect the year of registration in a straightforward way. The conventional format is for two letters, then a two-digit number and finally three more letters, “XX 21 XXX” for example. The 21 indicates a car registered between 1 March and 31 August 2021.

The licence plates for new cars registered on or after the other date in the year (1 September) reflect the year of issue in a less straightforward way: the two-digit year, plus 50. Before today the most recently registered cars had the number 70 on their licence plates, in this format: “XX 70 XXX”. Cars registered on 1 September 2019 would include the number 69, cars registered on that date in 2018 featured 68, and so on.

Over the years I have explained this system to my son. In recent weeks, on our once-a-day lockdown walks around local streets, he has been calculating the date of issue of many of the cars we pass, often commenting on how that relates to friends and family. 06 means the first half of 2006, so the car is older than his sister; 56 means the second half of 2006, probably also older than his sister (she was born in October). 61 indicates the second half of 2011, after the birth of his godfather’s daughter and his godmother’s son, born in July and August of that year respectively.

The date that this “new format” first appeared on licence plates was 1 September 2001. Later that month, returning from honeymoon, my wife and I caught sight of the first cars with these new plates and couldn’t work out what the letters and digits indicated. It was the Saturday after 9/11 and we were queuing to travel back to the UK, by car, through the Eurotunnel. We were anxious, like everyone else in Europe or America travelling from one country to another that week. Could the Channel Tunnel be the target for a terrorist attack? It felt possible. We noted that some of the British cars alongside us had number plates in the format “XX 51 XXX” but couldn’t check what that meant until we got home.

The first two letters of the new licence plates indicate the place of origin, in a way that only people in the motor trade understand. If it starts with an “L”, it indicates London. The three cars that we have owned since 2001 have had plates beginning LN, LS and LL, all registered here in West London. The digits 51, we learnt, indicate that the car was registered in the second half of 2001. The remaining three letters are random and have no significance regarding place of origin or year of registration. They offer 17,576 (or 26 cubed) different combinations, assuming that all 26 letters of the alphabet can be used.

The first customized plate that I saw using the digits “51” was in a printed advert, for something like car finance. It posed a question along the lines of, “Can I use [FinanceCo] to buy a brand new car?” The answer was in the form of a new registration number: YE 51 CAN (Yes, I Can).

Earlier today my son and I took a walk to see if anyone in the neighbouring streets had been quick off the mark and bought a new car on the first day of “21” licence plates. We didn’t see any, and I explained again how, before 2001, the format was different. The first year that licence plates indicated a year of registration was 1963, using the letter A at the end. These plates featured, typically, seven characters: three letters, three digits, another letter, “XXX 123 A” for example. From 1963 through to the end of 1966 the letters “A” to “D” indicated cars registered between 1 January and 31 December in each year. In 1967 that changed, and “E”-reg plates were issued between 1 January and 31 July. “F”-reg plates were issued between 1 August 1967 and 31 July 1968, and 1 August remained as the first date of issue for new plates through to 1998.

There is a car at the end of our road, an old Triumph Herald, and the plates have a G at the end, indicating that it was registered between August 1969 and July 1970. The first four cars that my mother owned, between 1971 and 1993, ended in the letters “H”, “L”, “R” and “Y”. The last of these was registered in 1982.

By 1983 every letter in the alphabet had been used up. “I”, “O”, “Q”, “U” and “Z” were not used, for one reason or another (similarities to numerical digits or other letters, although “S” had been included). From 1983 the format of number plates was reversed, as follows: A 123 XXX. The new-style “A”-reg plates were issued between 1 August 1983 and 31 July 1984.

When we got to “S”-reg cars (1 August 1998) there was another change, with two dates in the calendar for new registrations. These have remained the same (1 March and 1 September) since they were introduced in 1999. “T”-reg cars were registered between 1 March and 31 August 1999, the letter “U” was skipped again, and “V”-reg cars, cover 1 September 1999 to 29 February 2000. The last of the old-style plates were in the format “Y 123 XXX”, issued between 1 March and 31 August 2001.

If you want to see the dates and registrations in neat, tabular format, check out this page from the AA website (that’s the Automobile Association website, in case the letters “AA” mean something different to you).

Earlier tonight, while the rest of the family were watching the first episode of a new series of “Masterchef”, I took a longer walk than usual around the neighbourhood to see if there were any “21”-reg cars out there. A dozen streets later I spotted my first newly-registered car of the season, in a road parallel to our own. It was a new Kia, outside a house with a “Sale Agreed” sign, but it’s possible that the car and the house are not connected in any way.