Posted by Michael on October 21, 2001
In Reply to: P's and q's posted by R. Berg on October 16, 2001
: : : where does the expression watch your p's and q's come from and what does it mean
: There are different ideas about the origin. The link below connects to a previous discussion (http://www.phrases.org.uk/bulletin_board/5/messages/794.html). The phrase means behave yourself.
I understand that this comes from the time when typesetters put individual letters in place before using a press for newspapers, books, or whatever. P's and Q's looked the same and were very easy to reverse. Of course, it also means to behave yourself.