letters to dignitaries such as heads of state, ambassadors and ministers require the highest level of formality. For these addressees, WHO uses the formal closing 'Please accept, Sir (Madam), the assurance of my highest consideration’. This statement is followed by the signature but without 'Yours sincerely’. It is not our purpose here to show you all the possibilities for formality in letter-writing; rather, our intention is to make you aware that there are very specific requirements for many types of letters, and that being unaware of these formalities will delay the delivery of your message.

* 'Assuring you of our highest consideration and best cooperation at all times, I remain, Sir, your humble servant, J Smith, Esquire'
and:
* 'Yours sincerely, J Smith'

except that one takes four times as long to write and read as the other and uses up four times as much ink. I'm sure no modern-day head of state would object to a letter ending in 'Yours sincerely'. In fact, with politicians (as opposed to monarchs) I'm sure it's the norm.