Drawing upon her rich experience of life, Prudence (Prudie to her friends) responds to questions about manners, personal relations, politics, and other subjects. Please send your questions for publication to prudence@slate.com. Queries should not exceed 200 words in length. Please indicate how you wish your letter to be signed, preferably including your location.

Dear Prudence,

I can't believe that I'm writing to you, but I can't get this out of my mind. My very nice sister-in-law invited us to her son's high-school graduation dinner at a nice restaurant immediately after the ceremony. We agreed to go to both events, graduation and dinner.

My question is this. We had to leave early because of the baby sitter, so we got up, kissed the new graduate, said goodbye to everyone, and thanked our sister-in-law. A relative--not the host--said, "Leaving before the bill arrives?" If he knew the guests were supposed to pay for dinner, we didn't. Embarrassed, I gave my father-in-law money to give to the hostess to cover our meals.

Was I wrong to assume the dinner was given by my sister-in-law? What's the etiquette about being invited to a restaurant to celebrate a big event? I paid this time, but what to do next time? (And yes, we gave him a very, very nice graduation present.)

--Definitely Not a Freeloader in New York

Dear Def,

Prudie is appalled, and suspects your very nice sister-in-law was raised by wolves. At the very least, if the host can't manage such a party, guests should be informed beforehand that the celebratory dinner is the gustatory equivalent of BYOB.

As for the next time, feel free to ask if the party is a Dutch treat. If it is, and you're not feeling Dutch, decline with thanks.

--Prudie, choosily