Untangling this "lord" business, part II

"I’ll start with the sons of dukes and marquesses, since they’re more straightforward. Obviously, their names don’t change upon marriage. Lord Peter is still Lord Peter. But when he marries Harriet Vane (a commoner, daughter of a country doctor), she becomes Lady Peter. Not Lady Wimsey, because younger sons’ courtesy titles attach to the first name, not the surname. And not Lady Harriet, because it’s his title, not hers. Incidentally, their children don’t have titles, courtesy or otherwise."

Conversations With Dead People: Of Wimseys and Wellesleys: Courtesy Titles, Part 2