Be on your best behaviour; be careful of your language.

Ps and Qs are just the plural of the letters P and Q. There some disagreement amongst grammarians about how to spell Ps and Qs - either upper-case or lower-case and either with or without a hyphen. You may see the phrase as mind your p's and q's or mind your Ps and Qs or (occasionally) mind your P's and Q's or (rarely) as mind your ps and qs.

As well as the spelling, the original meaning is also in doubt. Francis Grose, in his 1785 edition of The Dictionary of the Vulgar Tongue, defines it like this:

"To mind one's P's and Q's; to be attentive to the main chance."

The date of the coinage of mind your Ps and Qs is uncertain. The OED used to print a citation from 1779 but, as they have now withdrawn it from the online version of the dictionary, presumably they consider it unreliable.

So, the meaning, spelling and coinage of the phrase are all debatable. Now we come to what is really uncertain - the derivation. Nevertheless, it is one of those phrases that people know the origin of. When pressed all that really means is that the person they heard explain the origin had made a random choice from the list of proposed derivations below. As no one knows the origin I'll just list those suggestions - mind your ps and qs probably derives from one of these:

- Mind your pints and quarts. This is suggested as deriving from the practise of chalking up a tally of drinks in English pubs (on the slate). Publicans had to make sure to mark up the quart drinks as distinct from the pint drinks. This explanation is widely repeated but there's little to support it, apart from the fact that pint and quart begin with p and q.

- Advice to printer’s apprentices to avoid confusing the backward-facing metal type lowercase Ps and Qs. I've never heard any suggestion that printer should mind their ds and bs though, even though that has the benefit of rhyming, which would have made it a more attractive slogan.

- Mind your pea (jacket) and queue (wig). Pea jackets were short, rough woollen overcoats, commonly worn by sailors in the 18th century. Perruques were full wigs worn by fashionable gentlemen. It is difficult to imagine the need for an expression to warn people to avoid confusing them.

- Mind your pieds (feet) and queues (wigs). This is suggested to have been an instruction given by French dancing masters to their charges. This has the benefit of placing the perruque in the right context - so long as we accept the phrase as being originally French. There's no reason to suppose it is from France and no version of the phrase exists in French.

- It is advice to children learning to write to take care not to mix up the lower-case letters p and q. Again, the 'd' and 'b' counter argument applies.

- It derived as reminder to children to be polite. This is supposed to be as a form of 'mind your pleases and thank-yous' - 'mind you pleases and kyous'. Pretty far-fetched that one.

- P and q stands for "prime quality." There is, or rather was as this now seems to have also been withdrawn, a 1612 citation which links PQ with 'prime quality'. If that's the origin why isn't the phrase mind your PQ?

So, pay nothing and take your choice. For what it's worth, my virtual two-penneth goes to the advice to children who were learning to write.