Statute of Westminster Day 11th December
On the 11th December 1931 the Statute of Westminster was signed into the British statute books.
… It is hereby declared and enacted that the Parliament of a Dominion has full power to make laws…
The act contained within it the requirement for the Dominions, later Realms, to seek agreement among each other on the laws of succession and titles.
It is this act that forms the basis of the sovereignty of the Realms and their relationship with the Crown.