Particle_Man said:I got prince for son of a king, baronet for son of a baron, what about the son of a duke?
Whoa there!
My uncle was a baron (life peerage). That made his eldest son "The honourable Dr. Jonathan Hunt", not a baronet. Baronetcies are independent hereditary titles, so far as I know never used as courtesy titles for eldest sons. By the way, baronetcies were invented in the late 17th century, they are not a mediaeval thing at all.
The rule is that the eldest son of a duke, marquis, or earl uses one of his father's lesser titles as a courtesy title, and the other children are (by courtesy) "Lord <firstname> <lastname>" or "Lady <firstname> <lastname>". All the children of viscounts and barons are "the honourable <firstname> <lastname>".
There isn't an actual English word meaning specifically "son of a duke" or "son of a baron", and those sons don't in the English system have titles either.