Mind your Ps and Qs

Several origins compete for this phrase about being careful and polite. One suggests it comes from English pubs, where bartenders reminded patrons to mind their pints and quarts. Another points to printing presses, where typesetters had to be careful not to confuse the mirror-image lowercase p and q. A third attributes it to French dancing masters reminding students about "pieds" (feet) and "queues" (wigs).