To guarantee the continuity of multiple states sharing the same person as monarch, the preamble of the Statute of Westminster 1931 laid out a convention that any alteration to the line of succession in any one country must be voluntarily approved by the parliaments of all the realms.[† 8] This convention was first applied to the abdication of Edward VIII in 1936. For expediency and to avoid embarrassment, the British government had suggested that the Dominion governments automatically regard the monarch of the UK, whoever this may be, as their monarch also. But the Dominions rejected this: Prime Minister of Canada William Lyon Mackenzie King pointed out that the Statute of Westminster required Canada's request and consent to any legislation passed by the British parliament before it could become part of Canada's laws and affect the line of succession in Canada. Sir Maurice Gwyer, first parliamentary counsel in the UK, reflected this position, stating that the Act of Settlement was a part of the law in each Dominion. Though today the Statute of Westminster is law only in Canada, Australia, and the United Kingdom, the convention of approval from the other realms was reasserted by the Perth Agreement of 2011, in which all 16 countries agreed in principle to change the succession rule to absolute primogeniture, to remove the restriction on the monarch being married to a Catholic, and to reduce the number of members of the Royal Family who need the monarch's permission to marry. These changes came into effect on 26 March 2015. Alternatively, a Commonwealth realm may choose to cease being such by making its throne the inheritance of a different royal house or by becoming a republic, actions to which, though they alter the country's royal succession, the convention does not apply.
Agreement among the realms does not, however, mean the succession laws cannot diverge. During the abdication crisis in 1936, the United Kingdom passed His Majesty's Declaration of Abdication Act with the approval of the parliament of Australia and the governments of the remaining Dominions. (Canada, New Zealand, and South Africa gave parliamentary assent later.) The Act effected Edward's abdication in the United Kingdom on 11 December; as the Canadian government had requested and consented to the Act becoming part of Canadian law, and Australia and New Zealand had then not yet adopted the Statute of Westminster, the abdication took place in those countries on the same day. The parliament of South Africa, however, passed its own legislation—His Majesty King Edward the Eighth's Abdication Act, 1937—which backdated the abdication there to 10 December. The Irish Free State recognised the king's abdication with the Executive Authority (External Relations) Act 1936 on 12 December. According to Anne Twomey, this demonstrated "the divisibility of the Crown in the personal, as well as the political, sense." For E H Coghill, writing as early as 1937, it proved that the convention of a common line of succession "is not of imperative force". and Kenneth John Scott asserted in 1962 that it ended the "convention that statutory uniformity on these subjects would be maintained in the parts of the Commonwealth that continued to owe allegiance to the Crown".
Today, some realms govern succession by their own domestic laws, while others, either by written clauses in their constitution or by convention, stipulate that whoever is monarch of the United Kingdom is automatically also monarch of that realm. It is generally agreed that any unilateral alteration of succession by the UK would not have effect in all the realms.[† 9]
Following the accession of George VI to the throne, the United Kingdom created legislation that provided for a regency in the event that the monarch was not of age or incapacitated. Though input was sought from the Dominions on this matter, all declined to make themselves bound by the British legislation, feeling instead that the governors-general could carry out royal functions in place of a debilitated or underage sovereign; some realms that were later created incorporated this principle into their constitutions. New Zealand included in its Constitution Act 1986 a clause specifying that, should a regent be installed in the United Kingdom, that individual would carry out the functions of the monarch of New Zealand.