Titles and Forms of Address, 20th edition, is a mine of information for a historical author. It becomes obvious on delving into it that the strange ways of the addressing nobility have a purpose, if not a secret code . Though I shall never be introduced to the Queen, I know that if she speaks to me, I should answer using her title Your Majesty for the first response and subsequently can get away with Ma'am. Should Prince Philip speak to me, I answer with his title Your Royal Highness and subsequently Sir.
If I meet the children of the Queen, the same rule applies - initial response Your Royal Highness and subsequently Sir or Ma'am.
The peerage has five grades - Dukes, Marquesses, Earls, Viscounts and Barons. A hereditary title descends from father to son or grandfather to grandson. Occasionally descent includes the female line. If a cousin succeeds in an ancient peerage, it is because he is descended from some former holder of the title, not because the previous peer was his cousin. With newer titles, it gets complicated over who might or might not inherit.
All peers have a family name as well as their title. Sometimes they are the same. Sons and daughters of peers use the family name, except in the case of eldest sons of dukes, marquesses and earls. The eldest son takes a courtesy title - in effect he borrows one of his father's lesser titles from the day he is born and uses it as his own.
It is a lot to remember when you are writing an exciting romance. More to come with the next post.