What is the origin of the term "Mind your P's and Q's"?

A website (http://alt-usage-english.org) on the usage of the English language provides the following explanation:

"This expression, meaning "be very careful to behave correctly", has been in use from the 17th century on. Theories include: an admonishment to children learning to write; an admonishment to typesetters (who had to look at the letters reversed); an
admonishment to seamen not to soil their navy pea-jackets with their tarred "queues" (pigtails); "mind your pints and quarts"; "mind your prices and quality"; "mind your pieds and queues" (either feet and pigtails, or two dancing figures that had to be accurately performed). Also, the substitution of /p/ for "qu" /kw/ in the speech of uneducated ancient Romans; or the confusion by students learning both Latin and Ancient Greek of such cognates as pente and quintus. And yes, we've heard the joke about the instruction to new sextons: "Mind your keys and pews."

The most plausible explanation is the one given in the latest edition of Collins English Dictionary: an alteration of "Mind your 'please's and 'thank you's"."

Maira, that was quite a wide range of possible answers, but the one I was thinking of is based on the fact that:

In old England ale is/was drunk in pints and quarts. So when customers got unruly, the innkeeper would yell at them to mind their own pints and quarts and settle down.

I have read most of the possibilities that Maira quoted, but there doesn't seem to be general agreement on any one origin. To me, the similarity of lower case p's and q's would seem the most likely, but who knows?

Degas, the explanation I've always heard is very similar to the one you cite, but seems more likely to me.

That is, that in the old days, publicans kept track of what you were drinking on a chalkboard, marking a "p" or "Q" next to your name for each one you ordered. Unscrupulous barkeeps could easily take advantage of patrons who were too busy elsewhere or simply "having too much fun"...so the wise tippler would mind his (own) p's and q's.

Hasa anyone else ever heard this explanation?

Yes, actually, I have heard that it was regarding pints and quarts.

Robdaddy, I like your version better.

I "liberated" mine off an official London Tourism site. Maybe the Brits didn't want to advance the shocking idea that they had pub owners who would cheap a customer!

Anyway, small tidbits like these make a trip more much interesting to me.