Leaders and representatives from across the Commonwealth are expected to come to London for the Coronation of King Charles III on 6 May.
Some of these countries were previously in the British Empire, some are still British dominions, while others had no colonial links to Britain but have chosen to join the global club.
Here are seven things you may not know about the Commonwealth.
About 2.5 billion people – out of a global population of eight billion – live in the 56 Commonwealth countries.
More than 60 per cent of the Commonwealth's population is aged 29 or under. Globally, a third of all young people aged between 15 and 29 live in Commonwealth countries.
The biggest country by population is India, which accounts for about half of the total.
Pakistan, Nigeria and Bangladesh are the next biggest countries by population, with the UK fifth.
King Charles is the head of state in 14 Commonwealth countries – or realms – in addition to the UK.
Most countries in the club are republics – with Barbados the last to remove the UK monarch as its head of state in 2021.
Five countries – Lesotho, Eswatini (previously known as Swaziland), Brunei Darussalam, Malaysia and Tonga – have their own monarch.
King Charles remains head of state in Australia, New Zealand and Canada, although for many years there has been an active movement in Australia in favour of becoming a republic. The country's central bank has announced that Charles will not feature on Australia's new $5 note.
Rwanda and Mozambique became members in 2009 and 1995 respectively, although neither were colonised by the British. Rwanda was previously under the rule of Germany and Belgium.
Former French colonies Gabon and Togo are the most recent joiners, becoming members in June 2022.
The club has also lost members.
South Africa withdrew in 1961 after it was criticised by Commonwealth members for its apartheid policies. It became a member again in 1994.
Pakistan was thrown out after a military coup in 1999, but was readmitted in 2004. Membership was suspended again between 2007 and 2008.
Former president of Zimbabwe Robert Mugabe took his country out in 2003 after its membership was suspended amid reports of election rigging.
It applied for re-admission in 2018, but no decision has been reached.
The last country to leave was the Maldives in 2016, but it re-joined in 2020.
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 member countries are small – such as the Pacific island nations of Nauru, Samoa, Tuvalu and Vanuatu, and Antigua and Barbuda, and Dominica in the Caribbean.
Thirty-three of the world's 42 small states are part of the Commonwealth.
The UK is no longer the biggest economy in the Commonwealth, according to the latest GDP numbers from the International Monetary Fund, with India overtaking it for the first time in 2022.
The combined GDP of the 56 members is more than $13tn (£10.8tn). That's more than twice the size of Japan ($5tn, £3.75tn), but some way behind the US at $23tn (£17.2tn).
Trade with the Commonwealth accounted for 9% of the UK's total trade in 2021 – around the same as the UK's total trade with Germany.
The UK exported goods and services worth £62.6bn to the rest of the Commonwealth, while imports from member states totalled £58.2bn
The modern Commonwealth was formed in 1949, after "British" was dropped from the name and allegiance to the Crown was removed.
Only three people have been head of the organisation – King George VI, Queen Elizabeth II and King Charles III.
It's not a hereditary role, although the Commonwealth accepted King Charles as its head in 2018, when he was Prince of Wales.
The organisation is run from London by its secretary-general, currently Baroness Scotland. She was re-elected to a second term in June 2022, despite facing criticism from some member states over her performance.
The other founding members were Australia, Canada, India, New Zealand, Pakistan, South Africa, and Sri Lanka.
The Commonwealth Charter was adopted in 2012, and commits 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 in the modern world.
Supporters argue the benefits which membership brings include developmental support and co-operation on international goals.
Russia and a number of other former members of the Soviet Union set up the Commonwealth of Independent States in 1991.
It aimed to coordinate economic, defence, immigration and foreign policy across the bloc, although this proved difficult in practice. Georgia withdrew in 2009 after Russia seized part of its South Ossetia territory. Ukraine withdrew its membership in 2018.
There is another group called the International Organisation of La Francophonie – an alliance of French-speaking countries which aims to promote the French language and increase mutual co-operation.
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