Of the many Royal Style and Titles Acts, the most constitutionally important was actually called the Royal and Parliamentary Titles Act. Passed on 12 April 1927, it marked the start of a change in the way the king and government in the United Kingdom related with the governments of the British Dominions in the British Empire. The Statute of Westminster 1931 carried the change to its logical conclusion which confirmed the independence of Commonwealth countries from the United Kingdom government.
The act itself changed the name of the parliament in London to Parliament of the United Kingdom of Great Britain and Northern Ireland, rather than "Parliament of the United Kingdom of Great Britain and Ireland" recognizing the change in its status after the creation of the Saorstát Éireann in 1922.
More importantly for the Commonwealth countries, the act permitted King George V to change the Royal Style and Titles as he deemed fit, and such a proclamation was issued on May 13, 1927.
This was the concept of "One Throne, Many Crowns" given practical effect, because it proclaimed that George V was not king United Kingdom of Great Britain and Ireland and of the British Dominions but rather of Great Britain, Ireland and the British Dominions
The change in wording is subtle, but creates a list of nations of which he is king rather than grouping those nations all together as if under one government.
Separating the crown of Great Britain from that of Ireland ended the right of the government in London to advise the king on actions to take regarding Ireland. The King of Ireland would only take advice from ministers in Dublin. The new Governor-General in Dublin also became a conduit between the King of Ireland and the government of Saorstát Éireann, and did not receive confidential instructions and documents from the London government.
Separating the crowns also means that changes to the succession must be agreed upon by all of the Commonwealth Realms, lest the personal union of Crowns be broken. Éamon de Valera combined Edward VIII's abdication on 11 December 1936 with a drastic limitation royal power in Ireland. The delay in passing the Executive Authority (External Relations) Act, 1936 meant that Edward VIII was King of Ireland until 12 December 1936.
In 1953 the Dominion governments agreed that the practice of separate titles should continue in the reign of the new Queen Elizabeth II. Each country adopted their own titles, the British act of parliament clearly stated that it applied only to the United Kingdom and those overseas territories whose foreign relations were controlled by the UK government
The two crowns concept almost came into crisis because of Conrad Black, newspaper publisher. He had been promised a life peerage by Tony Blair's government in London. However, Black was a Canadian citizen and so Jean Chrétien prime minister of Canada objected, as Canada has always opposed its citizens receiving peerages since the Nickel Declaration of 1919. Blair delayed carrying out the promise until Conrad Black became a naturalised British citizen. If he had insisted, the Queen of the United Kingdom would have had to grant the honour despite objections from the Queen of Canada
An Act to provide for the alteration of the Royal Style and Titles and of the Style of Parliament and for purposes incidental thereto.
17 Geo. 5. c. 4 [12th April 1927.]
BE it enacted by the King's most Excellent Majesty, by and with the advice and consent of the Lords Spiritual and Temporal, and Commons, in this present Parliament assembled, and by the authority of the same, as follows:—
1. It shall be lawful for His Most Gracious Majesty, by His Royal Proclamation under the Great Seal of the Realm, issued within six months after the passing of this Act, to make such alteration in the style and titles at present appertaining to the Crown as to His Majesty may seem fit.
2.-(1) Parliament shall hereafter be known as and styled the Parliament of the United Kingdom of Great Britain and Northern Ireland; and accordingly, the present Parliament shall be known as the Thirty-fourth Parliament of the United Kingdom of Great Britain and Northern Ireland, instead of the Thirty-fourth Parliament of the United Kingdom of Great Britain and Ireland.
(2) In every Act passed and public document issued after the passing of this Act the expression "United Kingdom" shall, unless the context otherwise requires, mean Great Britain and Northern Ireland.
3. This Act may be cited as the Royal and Parliamentary Titles Act, 1927.
PROCLAMATION by His Majesty the King altering the Style and Titles appertaining to the Crown.'
London, May 13, 1927.
WHEREAS by The Royal and Parliamentary Titles Act, 1927, it is enacted that it shall be lawful for us by our Royal Proclamation under the Great Seal of the Realm issued within 6 months after the passing of the said Act to make such alteration in the style and titles at present appertaining to the Crown as to us may seem fit;
And whereas our present style and titles are, in the Latin tongue, Georgius, V Dei Gratia Britanniarum et terrarum transmarinarum quae in ditione sunt Britannica Rex, Fidei Defensor, Indiae Imperator, and in the English tongue. George V by the Grace of God of the United Kingdom of Great Britain and Ireland and of the British Dominions beyond the Seas King, Defender of the Faith, Emperor of India;
And whereas we have received a recommendation from the representatives of our Governments, in conference assembled that our style and titles should he altered as in manner hereinafter appearing:
We have thought fit, and we do hereby appoint and declare, by and with the advice of our Privy Council, that: henceforth so far as conveniently may be, on all occasions and in all instruments wherein our style and titles are used, the following alteration shall be made in the style and titles at present appertaining to the Crown, that is to say, in the Latin tongue, for the word – Britanniarum - there shall be substituted the words – Magnae Britanniae, Hiberniae, and in the English tongue, for the words the United Kingdom of Great Britain and Ireland and of the words Great Britain, Ireland and.
Given at our Court at Buckingham Palace, this 13th day of May, in the year of our Lord 1927, and in the 18th year of our reign.
God Save the King.