How do you address a letter to someone with the title "Sir"?

When addressing a letter to someone with a knighthood, the socially accepted method of address is to include the title "Sir" before including their full name. This method of address only applies to nationals of the British Empire; foreign nationals are not entitled to use the formal prefix "Sir."

Under the British knighthood system, there are two types of knighthoods that can be granted. These are Knights of the Orders of Chivalry and Knights Bachelor. A knight belonging to a chivalric order carries letters after his name. The letters indicate which order he belongs to. A Knight Bachelor, on the other hand, does not carry letters after his name. A title of Knighthood is not inherited after death.