For future reference, I’ve included ways to address them all so that you need not worry about making a faux pas:

The Queen: Your Majesty and thereafter: Ma’am (rhymes with lamb)

The Duke of Edinburgh: Your Royal Highness, and thereafter: Sir

The Duke of York and the Earl of Wessex: Your Royal Highness, and thereafter: Sir

Prince William and Prince Harry: Your Royal Highness, and thereafter: Sir

The Princess Royal: Your Royal Highness, and thereafter: Ma’am

Princess Beatrice, Princess Eugenie, Princess Michael, Princess Alexandra, the Duchess of Kent, the Duchess of Gloucester: Your Royal Highness, and thereafter: Ma’am

Fairly straightforward, wouldn’t you agree?

Now, onto nobility. Here it becomes a bit more complicated. A veritable mine-field of potential offence. After all, what exactly are you supposed to do? Royalty is fairly recognizable, and you may know Earl Spencer (Diana, Princess of Wales’ brother) but would you be able to pick out the Duke of Wesminster in a crowd? Once again, when in doubt, err on the side of caution. Especially if no one is wearing a name tag.

There are five forms of Peerage: Duke, Marquess, Earl, Viscount, Baron.

Duke: Your/His Grace

Marquess: My Lord

Earl: My Lord

Viscount: My Lord