Mind your P's and Q's

Mind your P's and Q's: Sailors would get credit at the taverns in port until they were paid. The barman would keep a record of their drinks on a chalkboard behind the bar. A mark was made under "P" for pint or "Q" for quart.  On payday, the sailors were liable for each mark next to his name, so he was forced to "mind his P's and Q's." Today the term means to remain well behaved.