When I was about 12 years old, I read a children's book about manners. There might have been a much more intricate story but the thing that stuck with me was an included anecdote about minding one's P's and Q's.
I don't know if this little tale is true, but it made sense to me when I read it so I might as well share it. Not very long ago the phrase, "mind your p's and q's" was common among mothers, teachers and anyone else who kept children in line. The story goes that the P refers to the beginning of PLEASE and the Q refers to the sound that is made in the middle of THANK YOU (THAN-Q). Therefore, when a child is told to mind her P's and Q's it is a reminder to say please and thank you.
I always thought that the saying meant more than using the 'magic words' but related to the broader concept of etiquette and conformity to social convention. The context in which adults seemed to use the phrase indicated to me that grave consequences were brought upon anyone whom dared to forget a P or Q.
Did anyone stop to think that "p" and "q" look a like and it might just be a reminder that concentration while handwriting is imperative. A good proof reader will rectify any careless errors related to the mixing up of p's, q's, b's and d's in any case, I suppose.
Only a portion of this was relevant to the original purpose of this post when I started typing. However, I think you have all learned something valuable... even if that is only "don't believe everything you read on the Internet".
By the way, if you are interested in the true origin of the P's and Q's saying click here to see that there is no real agreement on the matter at all.
I will simply have to write another post.