Barbados has removed Queen Elizabeth II as its head of state to become the world's newest republic - but it will remain within the Commonwealth.
Here are some things about this club of more than 50 nations that you might not know.
1) It's home to almost one-third of the world's population
About 2.6 billion people - out of 7.9 billion globally - live in the Commonwealth's 54 countries.
The biggest country by population is India, which accounts for about half of the total.
Pakistan, Nigeria and Bangladesh are the next biggest by population, with the UK fifth.
2) Some members were never part of the British Empire
Rwanda and Mozambique became members in 2009 and 1995 respectively, and neither were colonised by the British.
The club has also lost members in the past.
Former president of Zimbabwe Robert Mugabe took the country out in 2003 after its membership was suspended amid reports of election rigging.
It applied in 2018 to be readmitted, but no decision has been reached yet.
Pakistan was suspended after a military coup in 1999 and was re-admitted four-and-a-half years later.
And South Africa withdrew in 1961 after it was criticised by Commonwealth members for its apartheid policies. It became a member again in 1994.
The last country to leave was the Maldives in 2016, but it rejoined in 2020.
3) The Queen is head of state in only 15 of the countries
Most of the Commonwealth states are republics, with Barbados becoming the latest, having decided to remove the Queen as head of state.
Five - Lesotho, Swaziland, Brunei Darussalam, Malaysia, and Tonga - have their own monarch.
The Queen remains head of state in Australia and Canada, although there's an active movement in Australia in favour of becoming a republic.
4) It's big
The Commonwealth makes up a quarter of the world's land mass.
The giant of the group is Canada, the world's second largest country by area. India and Australia are huge too.
But many of the states are small - like the Pacific island nations of Nauru, Samoa, Tuvalu and Vanuatu, and Dominica and Antigua and Barbuda in the Caribbean.
5) The UK still has the biggest Commonwealth economy… just
The UK has the biggest economy in the Commonwealth, according to the latest GDP numbers from the International Monetary Fund.
But India is predicted to overtake it in the next few years.
The combined GDP of the 54 countries is more than $10tn (£7.5tn). That's twice the size of Japan ($5tn, £3.75tn), but some way behind that of the US on $23tn (£17.2tn).
Trade with the Commonwealth accounted for 9.1% of the UK's total trade in 2019 - around the same as the UK's total trade with Germany.
UK exports to the Commonwealth were worth around £65 billion, and imports from the Commonwealth were around £64 billion.
6) It changed its name
The modern Commonwealth was formed in 1949, after "British" was dropped from the name and allegiance to the Crown was removed.
Only two people have been head of the organisation - King George VI and Queen Elizabeth II.
It's not a hereditary role, although the Prince of Wales is widely expected to take it up when he becomes king.
The founding Commonwealth members were Australia, Canada, India, New Zealand, Pakistan, South Africa, Sri Lanka and the United Kingdom.
The Commonwealth Charter was adopted in 2012 - which committed members to the values of democracy, gender equality, sustainable development and international peace and security.
The Commonwealth has been criticised for being a post-colonial club and for having little influence.
Supporters say the benefits which membership brings include developmental support and co-operation on international goals.
7) There's more than one commonwealth
There's the Commonwealth of Independent States, which was set up in 1991 by Russia and other former members of the Soviet Union.
And don't forget the International Organisation of La Francophonie - a group of French-speaking countries which aims to promote the French language and increase mutual co-operation.