And the American pint isn't the same as the imperial pint anyway. It's a bit smaller.

That's a good story about the P's and the Q's, but I suspect that the term goes back further than that. Though in the 1969 edition of Brewers Dictionary of Phrase and Fable, a version of that is one of the possibities given "More fancifully it is suggested that in public houses accounts were scored up for beer P for pints and Q for quarts, and a customer needed to mind his P's and Q's when the reckoning came."

Another suggestion in Brewer is that it comes from courtiers in Louis XIV time, who had to be carefull when bowing in case their wigs fell off - which meant minding their pieds (feet) and their queues (wigs).

But "most probably it derives from an admonition to children learning the alphabet to be careful to distinquish between the forms of p and q, or to printers' apprentices in handling and storing type."