Why King Charles and Queen Camilla sign their names with the letter ‘R’ as people spot detail in latest post

Royal fans are just learning why the King and Queen use the letter 'R' when signing something off, as the old tradition was used on their most recent post.

While you would be forgiven for thinking the random letter 'R' used after the King and Queen's names stands for 'royal', fans are only just working out what it actually stands for.

In their most recent post, King Charles and Queen Camilla shared that they are spending their twentieth wedding anniversary in Italy, more specifically Rome and Ravenna.

Taking to social media to share the update, the royals wrote: "As we begin our first visit to Italy as King and Queen, we are so looking forward to celebrating our twentieth wedding anniversary in such a special place - and with such wonderful people! A presto, Roma e Ravenna! Charles R & Camilla R."

The couple are celebrating their 20th wedding anniversary (Chris Jackson/Getty Images For Clarence House)

But many people in the comments were confused by the random letter at the end of their names.

It turns out the 'R' stands for 'Rex and Regina' - Regina translates to Queen in Latin, and Rex translates to King.

"R is for Rex?" one person questioned in the comments.

While another wrote: "Rex and Regina. Well spotted!"

The tradition of using this initial can be dated back through centuries of the British monarchy.

Speaking to Harpers Bazaar, historian Marlene Koenig said: "It designates the authority of the sovereign.

"You will find the use of 'R' as far back as Henry I [who ruled in the early 12th century]."

The letter 'R' stands for 'Rex and Regina' (The Palace/Chris Jackson)

He added: "It is how they sign all official documents, as well as formal letters, etc."

Like Camilla, who is technically the Queen Consort, spouses of the sovereign have also used the initial to indicate their position.

But Queen Elizabeth's late husband, Prince Philip, decided against using the letter 'R'.

He ditched the tradition completely, and opted for something even more casual - signing his name as simply just 'Philip'.

But it isn't the only tradition that royal fans have been working out in recent years.

It's been made known that the institution isn't allowed to eat shellfish, and that the royal ladies must wear tights on certain occasions.

Other traditions include heirs traveling separately when flying for safety, and also carrying with them a black mourning outfit in their suitcase, just incase a member of the family dies.