PA Photos / Landov Queen Elizabeth II, left, and Michelle Obama at Buckingham Palace

The rules are set in stone, and so the eagerly watching British media sputtered when the First Lady of the United States, Michelle Obama, briefly put her hand on the back of Queen Elizabeth II as the two chatted at a reception. Etiquette is quite stern about this ("Whatever you do, don't touch the Queen!"). In 2000 John Howard, then Prime Minister of Australia, got plenty of criticism for apparently putting his arm around the Queen to direct her through a crowd. He denied actually touching her, but photographs suggest that he came quite close. (Another former Australian Prime Minister did put his hand on the Queen in a similar circumstance and was later branded "the Lizard of Oz.")

Of course, there are corollaries to this. One must certainly touch the Queen if the monarch offers her hand (though you should return this not with a firm handshake but just a touch). On Wednesday, Michelle Obama put her hand on the Queen only after the Queen had placed her own hand on the First Lady's back as part of their conversation. So there is room for theological argument as to whether the American reciprocity of touch was allowable given the social dynamics of the situation. (Less explicable was when President George W. Bush winked at the Queen.) Still, the sight of anyone apparently touching the Queen with anything more than a limp handshake is enough to send the British (or traditionalists in the old Commonwealth) twittering. (See pictures of the Obamas' travels in Europe.)

Another defense for Michelle Obama, of course, is that she is not a subject of the Queen. (Australians, despite referendums attempting to turn themselves into a republic, still recognize the Queen as their head of state.) The First Lady of the United States is not required to curtsey before her or any other crowned head. In any case, the touch lasted just a second or two, and the Queen did not seem particularly perturbed  though she appeared slightly surprised as she drew away. (See how Barack Obama is connected to the Queen via TIME's Person of the Year.)

So where does this rule about not touching the Queen come from? The sovereigns of England and France at some point in their nations' long histories claimed a divine right to rule, a right often amplified by titles bestowed by the Pope in Rome. (The Queen, in fact, still has the title Defender of the Faith, an honor given to Henry VIII before he broke with the Catholic Church and established the Church of England.) That touch of holiness once gave the occupant of the throne the supposed ability to cure certain diseases  most famously, scrofula, a terrible skin ailment that was called "the king's evil." Thus, the miraculous contact had to be conserved. And so, whether a touch or a nod or a gaze, royal favor, like that of God, is not a subject's on demand; it is dispensed by kingly prerogative. (See pictures from the 2006 celebration of the Queen's birthday.)

 With reporting by Simon Robinson / London

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