Mind Your "P's" and "Q's"

When I'm researching, I come across many articles of interest as I'm sure you do. Did you know that way back, at local taverns, pubs and bars, people drank from pint and quart sized containers? A bar maid's job was to keep an eye on the customers and keep the drinks coming. She had to pay close attention and remember who was drinking in "pints" and who was drinking in "quarts", hence the phrase "minding your "P's and Q's". This might be interesting to insert in a story where it might be appropriate.