HM the Queen's role as Head of the Commonwealth is symbolic, carrying no constitutional functions. Her relationship with Commonwealth countries is a warm and personal one, which enables her to act as a focus for the association.
The London Declaration, issued in 1949 after a Commonwealth Prime Ministers' meeting, recognised the King as the symbol of free association of the Commonwealth members and, as such, Head of the Commonwealth. This allowed India to remain within the association as a republic and paved the way for other countries which became republics on, or after, independence to remain within the Commonwealth. Previously, membership had been based solely on allegiance to The Crown.
This set the pattern for the development of the modern Commonwealth. While recognising HM the Queen as Head of the Commonwealth, members can have different constitutions, for example as republics, indigenous monarchies, sultanates, elected chieftaincies or as realms where HM the Queen remains Head of State. HM the Queen uses a personal flag - initial E and crown with a chaplet of roses - to show that her role as Head of the Commonwealth is different from her role as Head of State in Britain, the 15 other Commonwealth realms and related territories.
As HM the Queen declared in a speech marking her Silver Jubilee in 1977, the Commonwealth symbolises 'the transformation of The Crown from an emblem of dominion into a symbol of free and voluntary association. In all history this has no precedent.'
In 1995, the Commonwealth welcomed new members, Cameroon and Mozambique (a special accession as the only state not to have former ties to The Crown). HM the Queen has visited every Commonwealth country except the two new members. Altogether, one-third of HM the Queen's overseas visits are to Commonwealth countries. In 1997, her visits included Canada to join in the 150th Canadian National Day celebrations, and India and Pakistan to mark the 50th anniversary of their independence.
HM the Queen maintains close and personal links with the Commonwealth in many other ways, for example: regular contact with the Secretary-General and Heads of Government; the annual Commonwealth Day Observance Service in Westminster Abbey in London; broadcasts of her Christmas and Commonwealth Day messages; patronage of Commonwealth organisations and events; and often through opening and closing ceremonies at the four-yearly Commonwealth Games.
At the Heads of Government meetings, HM the Queen is usually present in the host country and has private meetings with Commonwealth leaders. She also gives a reception and dinner during the conference period at which she makes a speech.
The British Monarchy website