LONDON (Reuters) ? The Anglican Communion, facing schism over the issue of gay clergy, is the third largest family of Christians in the world after the Roman Catholics and the Orthodox.

Anglicanism traces its roots back to early Christianity but became a separate movement in 1534 when England's King Henry VIII broke from Rome to divorce Catherine of Aragon. It spread throughout the British Empire and beyond and is now present in 164 countries. It is called the Episcopal Church in Scotland and in the United States.

Here are five facts about the Anglican Communion:

The Anglican Communion has 77 million members in 38 churches or provinces loosely linked by four "instruments of unity." Provinces are self-governing and hold differing views on issues such as letting women ? or, in the case of the Episcopal Church in the United States, an openly homosexual cleric ? become bishops.

The four instruments of unity are the Archbishop of Canterbury, the decision-making Anglican Consultative Council, the 10-yearly Lambeth Conferences and the more frequent Primates' Meetings.

The Archbishop of Canterbury calls the Lambeth Conference and heads the Anglican Consultative Council. He is a "first among equals" in the Communion but does not have doctrinal or organisational powers such as those of a Roman Catholic pope.

Anglicanism is a "broad church" with elements common both to Catholicism (liturgy, sacraments, bishops) and Protestantism (more freedom for doctrinal interpretation, married clergy, decentralised structure). Its links to Catholic tradition have traditionally fostered dialogue with the Vatican, but the issues of women clergy and gay bishops have strained relations in recent years.

The growth of Anglicanism in developing countries has boosted the influence of the "Global South." The Anglican Communion's largest provinces are England (26 million), Nigeria (17 million), Uganda (8 million), Sudan (5 million), Australia (3.9 million) and Kenya (3.5 million). Regular church-goers in Africa easily outnumber those in Britain.