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It was once used (without the person's name) as a courtesy title among equals, but is now usually reserved for one of superior rank or stature (e.g. a teacher, a monarch or military officer); as a form of address from a merchant to a customer; in formal correspondence; or to a stranger (Sir, you've dropped your hat.) The equivalent for a woman is madam.

Sir is also the correct styling for a knight, used with the knight's given name or full name, but not the surname alone (Sir Paul McCartney or Sir Paul, not Sir McCartney). The equivalent for a woman is Dame.

It derives from the French sieur, meaning "lord," ultimately from the Latin senior.