Dukes in the United Kingdom
Duke, in the United Kingdom, is the highest-ranking hereditary title in all four peerages of the British Isles. A duke thus outranks all other holders of titles of nobility (marquess, earl, viscount and baron).
Forms of address in the United Kingdom
"Eldest sons of dukes, marquesses and earls use their father's most senior subsidiary title as courtesy titles: note the absence of "The" before the title. If applicable, eldest sons of courtesy marquesses or courtesy earls also use a subsidiary title from their (great) grandfather, which is lower ranking than the one used by their father. Eldest daughters do not have courtesy titles; all courtesy peeresses are wives of courtesy peers."
"(Heirs-apparent and heirs-presumptive of Scottish peers use the titles "Master" and "Mistress"; these are substantive, not courtesy titles. If, however, the individual is the eldest son of a Duke, Marquess or Earl, then he uses the appropriate courtesy title, as noted above.)"