Elbows off the table.
My wife is an absolute stickler for table manners. No elbows on the table, napkin on laps, prayer before you touch any food, proper use of utensils, mouth closed when chewing, sit straight, feet down, pleases & thank yous, ask do not reach, permission to be excused, etc., etc. It's interesting when our children have guests over for a meal and they find out that every time we sit to break bread it's basically a formal affair. My wife has no qualms about correcting anyone (child or otherwise) who dines at her table. She believes good manners are a sign of self-respect, dignity and class, that having those qualities at the table equates to having those qualities in life. Qualities that lend to more successful grown ups. And as she always says "I am not raising children, I am preparing future productive adults".