Where does the phrase, "Mind your P's and Q's" come from and what does it mean?

In English pubs, ale was ordered by pints and quarts. So, in old England, when customers got unruly, the bartender would yell at them to mind their own pints and quarts and settle down.

I don't know what the phrase means.

Literally, it means "mind your pints and quarts".

i always wondered that

I'm assuming it comes from how a lower case p looks like a reversed q and is an idiom which means whatch out for the small details. My idiom book says it comes from "Mind you toupees and queues" a queue being a pigtail populat in the 17th century. Another possible origin is how bar keeps used to hang a slate with numbers of P's or Q's written next to the name of customers (pint or quart)according to how much the customer had drunk.

Lower case ps and qs look similar. They are next to each other alphabetically. Children might easily confuse the two, particularly if they were dyslexic. Printers using movable type might sort them back into the wrong tray if they weren't paying close attention.

I don't know. Make sure you write them down correctly? Cos they look the same but mirrored?

Performances and Qualities?

i always wondered what they meant by that - cool!

I *know* this! I know that I know this. I just can't recall it right now.....

I've only ever heard it as an admonishment for swearing, not to be careful. I assumed it had some origin in blasphemy.

Pints & Quarts. Mind 'em so you don't get all sloppy drunk and do something that'd embarrass your mum.

p q p q pq
see?
easy to transpose and write a p backwards and get q or a q backwards and get p.
So pay attention when you write as not to reverse them and get all mixed up.

Mothers usually told their children to mind their p's and q's since the lower case letter p looks quite similar to the lower case letter q. Basically, it meant to pay close attention to detail, whether in speach or behavior.

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); 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, I'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".
http://www.alt-usage-english.org/excerpts/fxmindyo.html