'ER' or 'EIIR' is the Royal Cypher used for the late Queen that appears on everything from post boxes to police helmets and passports.
MORE: The touching meaning behind the Queen's funeral flowers and King Charles III's input
But now that Charles is King, what will the initials change to and what do they stand for?
WATCH: What are Charles's duties and powers as King?
What does ER stand for?
'ER' or 'EIIR' stands for Elizabeth Regina - Her Majesty's first name followed by the Latin word for Queen. The 'II' is to symbolise Elizabeth II.
This will now change to 'CR' or 'CIIIR' which stands for Charles Rex - His Majesty's first name followed by the Latin word for King. The 'III', again, symbolises that he is Charles III.
SEE: The Queen's state funeral: Royal family unite in grief - live updates and photos
MORE: King Charles III's coronation: when will it be and what will happen?
It was previously speculated that Charles was considering ruling under the name 'George VII' rather than choosing his birth name to avoid the negative connotations attached to Charles I, who is the only British monarch to have been publicly tried and executed for treason at the end of the Civil War in 1649. However, according to The Guardian, Clarence House denied these claims back in 2005.
'ER' will change to 'CR'
What will happen to passports and post boxes?
British passports currently reference the late Queen in the following way: "Her Britannic Majesty’s Secretary of State requests and requires in the name of Her Majesty all those whom it may concern to allow the bearer to pass freely without let or hindrance and to afford the bearer such assistance and protection as may be necessary."
This will be changed and updated with the King's name. However, this will only happen when passports need to be renewed as they approach their expiry date.
© Photo: RexPost boxes will remain unchanged
As for post boxes, they will not change. Royal Mail said: "In line with past practice, following the death of a monarch all existing post boxes will remain unchanged. Postboxes already in production or being prepared for installation, will also retain the insignia of Queen Elizabeth II."
Make sure you never miss a ROYAL story! Sign up to our newsletter to get all of our celebrity and royal news delivered directly to your inbox.
