If you've ever hung around a post box in the UK, really taking your time to savor the moment of sending in that tax return, you may have noticed a few letters on it, beneath an image of a crown.
If that's happened to you, you are not alone. Every now and then, someone spots them and asks for help solving the mystery.
The letters generally either say "G R" or "E R", or "G V R" or E II R". Sometimes, the letters are combined into one mess of a symbol, and if you're lucky, you can spot boxes with "V R" on there as well. If you're aware of the line of succession, you can probably already guess that the next post boxes erected in the UK will read "C R" or "C III R".
"The most common Royal Cypher to many would be that of [Her Majesty] Queen Elizabeth II," The Postal Museum explains. "Her ‘EIIR’ cypher stands for ‘Elizabeth II Regina’. The ‘R’ was added to a monarch’s cypher from the reign of Henry VIII, and stands for either ‘Rex’ or ‘Regina’, which is Latin for King or Queen."
Accordingly, "G R" stands for King George V and "V R" for Queen Victoria. As such, we will soon see postboxes with "C R", for King Charles III's reign.