Article: The British art of merger

In flags and coat of arms, a merger is usually a political act: it's used to represent the cooperation of two different parts or parties in condition of equality. I like to say it's a British art, for the many occasions it occurred on those lands. See some examples:

Royal badges of England

From 1455 and 1485, the then Kingdom of England lived a dynastic indecision between two rival branches of Plantagenet dynasty: the House of York and the House of Lancaster. The symbol of the House of York was a white rose pointing the bottom, while the House of Lancaster used a red rose pointing the top as insignia. When Henry Tudor (future king Henry VII) unified the two factions and achieved the peace, a new rose was created: the Tudor rose, still used as the royal badge of England.

York rose	Lancaster rose	Tudor rose

Royal badges of United Kingdom

So the Tudor rose became the royal badge of England. After the death of Queen Elizabeth I, the last monarch of Tudor dynasty, there were the Union of Crowns (1603) i.e. the King of Scots (the formal name of the King of Scotland) became automatically King of England. The kingdoms were administered separately, although had the same head of state, until the Acts of Union (1706-1707), that created the United Kingdom of Great Britain, constituted by the two former kingdoms.

The two kingdoms had particular pre-Union histories and, although the capital of new kingdom would be on London (England), the Scots wanted equal representation. A new badge was created, merging the Tudor rose, royal badge of England, and the thistle, the royal badge of Scotland.

Tudor rose (England)	Thistle (Scotland)	Royal badge of United Kingdom of Great Britain

In 1800, a similar merge occurred: the Kingdom of Ireland, that was in personal union with Kingdom of England since the Crown of Ireland Act (1572) and, after the Union Act, with the United Kingdom of Great Britain, was unified with the United Kingdom, by acts also name Acts of Union. The badge of the two kingdoms were also merged.

Royal badge of United Kingdom of Great Britain	Shamrock (Ireland)	Royal badge of United Kingdom of Great Britain and Ireland

The badge didn't change after the independence of Irish Free State (current Republic of Ireland) or the power devolution to Wales, that has its own badges.

Flags of United Kingdom

And the same things that occurred on the royal badges occurred on flags: the St. George's cross, the flag of England, was merged to the St. Andrew's cross, the flag of Scotland, to create the flag of the United Kingdom of Great Britain, that was merged to St. Patrick's cross, the cross-flag of Ireland, to create the flag of United Kingdom of Great Britain and Ireland. There are many proposals to add Welsh representation under the badge, but none approved.

St. George's cross (England)	St. Andrew's cross (Scotland)	Flag of United Kingdom of Great Britain
Flag of United Kingdom of Great Britain	St. Patrick's cross (Ireland)	Flag of United Kingdom of Great Britain and Ireland

Your comment is welcome!

Maybe, on a future opportunity, I explain the meaning of each one of the flags above: St. George's, St. Andrew's and St. Patrick's crosses.