POLITENESS
bread-and-butter letter
a letter or note written to follow up on a visit; a thank you note. (Fixed order.)When I got back from the sales meeting, I took two days to write bread-and-butter letters to the people I met.I got sort of a bread-and-butter letter from my nephew, who wants to visit me next summer.
mind one's p's and q's
to mind one's manners. (Fixed order.)When we go to the mayor's reception, please mind your p's and q's.I always mind my p's and q's when I eat at a restaurant with white tablecloths.
on one's best behavior
being as polite as possible.When we went out, the children were on their best behavior.I try to be on my best behavior all the time.