mind your p's and q's

Behave yourself. Opinion is divided over what the letters in the admonition to “mind your p's and q's” mean. They might have been two similar-looking letters that typesetters were warned not to confuse. They might have stood for pints and quarts that tavern keepers could have confused. Neither, however, is close to the idea of being on your best behavior, but p standing for “please” and q for “thank you” (pronounced thank-kew) does. And that's how generations of parents reminded their children to remember their manners.