Mind your own P's and Q's

[blueeye]

I've been rading Roald Dahl's The Magic Finger to my kid. There is an interesting idiom in the following sentence:

The boys laughed and made faces at me, and Mr Gregg told me to go home and mind my own P's and Q's.

Well, obviously it means to mind your own business, but I'm interested in origin of the phrase. Can anyone help me?

Thank you.

[thedaffodils]

I am not a teacher.

Mind one's p's and q's refers to:

• to behave properly; to display good manners
• to pay close attention to small details

Reference source:
mind p's and q's - Idioms - by the Free Dictionary, Thesaurus and Encyclopedia.

The letters of p and q should be in lower case, or the idiom wouldn't make sense. I think p and q look quite similar to each other, and people tend to mix them, so people have to pay close attention.

[Kondorosi]

I am not a teacher.

Mind one's p's and q's refers to:

• to behave properly; to display good manners
• to pay close attention to small details

Reference source:
mind p's and q's - Idioms - by the Free Dictionary, Thesaurus and Encyclopedia.

The letters of p and q should be in lower case, or the idiom wouldn't make sense. I think p and q look quite similar to each other, and people tend to mix them, so people have to pay close attention.

[Barb_D]

In addition, "in the old days" type was set on a press and it was backwards, so the p looked like a q and vice versa. You had to really pay attention to make sure you used a q where you wanted a q, because the p would have looked like a normal q.