This post was up for at least a few hours last week but disappeared during Blogger's issues, so I'm reposting it now.
First of all, a quick clarification from my previous post about heirs apparent vs. heirs presumptive. I stated that an heir apparent is the eldest legitimate son, or occasionally the eldest son's eldest son, of the current title holder. I should've said eldest living legitimate son. If a duke has two sons, and the elder dies without fathering a legitimate son of his own, the younger son then becomes the heir apparent. They key is that the heir apparent has to be the direct descendant of the current title holder, who will definitely inherit as long as he doesn't predecease him. If the current peer is your brother or uncle, you can only be the heir presumptive, because it's at least theoretically possible you could be displaced by your brother or uncle's son.
But...if you are the heir apparent of a duke, marquess, or earl, you probably have an heir's courtesy title derived from one of your father's subsidiary titles. High-ranking peers almost always have more than one title, generally through accumulating lower-ranking titles on their family's climb through the ranks of the peerage. So the peer's heir apparent bears the next highest of his father's titles and is addressed accordingly. And if the heir apparent has an eldest son of his own, he gets the next title on the list as his courtesy title, and so on.
It's probably simpler to give an example, and the currently living generations of the Wellington family provide an extensive and tidy one:
- The current Duke of Wellington (the 8th duke) was born in 1915 (the Wellesleys are a long-lived bunch--"the" Wellington lived to be 83, pretty impressive for his time).
- His eldest son, the current heir apparent, was born in 1945 and is styled the Marquess of Douro.
- Lord Douro's eldest son, born in 1978, is styled the Earl of Mornington. If you've been following this series and have a good head for details, you'll recall that was the title of the original Duke of Wellington's father. Wellington's oldest brother had no legitimate children, and his second brother's legitimate male line died out at some point in the 19th century (YES, I'm feeling too lazy to look up the exact date--there are limits to even my pedantry), so the title fell to the Wellington branch of the family.
- Last year Lord Mornington's wife had twins, one of whom is a boy, styled Viscount Wellesley.
Probably a good thing they have so many titles to go around, since all the men above are named Arthur Wellesley. (Of course they are. Feel free to hum "Tradition.")
Note that there is no rule that a duke's heir apparent must be a marquess, that a marquess's must be an earl, and so on. In our fictional example, Lord Peter's nephew, the heir apparent of his brother the Duke of Denver, is Viscount Saint-George. The heir's courtesy title is just the next highest ranking of his father's titles, whatever that happens to be.