It is the so-called special relationship - but over the years US presidents and first ladies have struggled to navigate the delicate dance of royal etiquette.
Donald Trump was the latest commander-in-chief to raise regal eyebrows on his state visit to Windsor Castle this week.
Critics claimed the 79-year-old breached 'unofficial rules of personal diplomacy' by complimenting the Princess of Wales on her appearance.
President Trump was recorded offering a handshake to Catherine, saying: 'You're beautiful. So beautiful'.
Later, at a formal state banquet on Wednesday, he described her again as 'so radiant, and so healthy, and so beautiful.'
