Today we look into what happens when the daughters and younger sons of last month's post get married.
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. They're just very well-bred and well-connected commoners.
Daughters of dukes, marquesses, and earls are more complicated. The rule, as I understand it, is that when a woman with a courtesy title marries, she takes her husband's title if it's higher in precedence than hers, but keeps her own if hers is higher. This means that if she marries a man without a title, she doesn't become Mrs. Husband's Name. Instead, she's Lady HerFirstName HisLastName. In the case of the fictional Wimsey family, Lord Peter's sister, Lady Mary, marries Charles Parker, a police detective and one of her brother's closest friends, and becomes Lady Mary Parker. (NOT Lady Parker.) Her husband's name doesn't change. Basically, a man can convey his courtesy title to his wife, but a woman can't pass hers to her husband. Is this sexist? Of course it is. If you're going to write historical fiction, you'll run into far worse instances of it.
Wellington's one sister also married commoners (untitled gentlemen might be a better term), becoming first Lady Anne FitzRoy (wife of the Hon. Henry FitzRoy, son of a baron) and then after his death Lady Anne Smith (wife of Charles Culling Smith).
If Lady Anne or Lady Mary had married a peer (a holder of an actual rather than a courtesy title), then she would've taken his title even if it were a lower rank of the peerage (baron, viscount), because substantive title always trumps courtesy title. I.e. if Lady Mary Wimsey had married Baron Stuffy, she'd be Lady Stuffy rather than Lady Mary.
It's when courtesy title holders marry each other that things get really complicated. Then, if I understand this correctly, you go by their relative precedence. Dukes' daughters trump dukes' younger sons, who in turn trump marquesses' daughters who trump their younger sons and so on. So Lady Mary Wimsey, as a duke's daughter, has precedence over any man's courtesy title. Therefore, no matter who she marries, unless he has an actual title of his own, she'll outrank him and continue to go by Lady Mary. But if Lady Anne Wellesley, an earl's daughter, had married the younger son of a duke or marquess, she would've become Lady HisFirstName HisLastName, because dukes and marquesses trump earls.
At least, I'm almost sure that's how it works. But I'm open to correction if not.