As I understand, Crown Princess Mary of Denmark was born Mary Elizabeth Donaldson of Australia. In order to marry Crown Prince Frederik, she had to renounce her Australian and Tanzanian citizenships, become a Danish citizen, and join the Evangelical Lutheran Church of Denmark (having been raised as a Presbyterian).

Does this mean that until the moment of her renunciation, she was technically and legally a subject of Her Majesty the British Queen? Of course, she would have automatically become a subject of Her Majesty the Danish Queen (soon to become her mother-in-law), after becoming a citizen of Denmark.

I'm not sure if Mary acquired Danish citizenship immediately after renouncing her Australian. But if so, this would have made an interesting scenario whereby at one moment, she was a subject of one queen regnant; at the next, she became the subject of another.

I'm not sure if HRH the Danish crown princess has encountered HM the British queen since her marriage; but one presumes that she has had no obligation to bow or curtsy to her.