Various, Notes and Queries, Vol. IV, Number 88…
“ In sorting pie, or a mixed heap of letters, where the "p" and the "q" are not in connexion with any other letters forming a word, I think it would be almost impossible for an inexperienced person to say which is which upon the instant. "Mind your p's and q's"—I write it thus, and not "Mind your P's and Q's"—has a higher philosophy than mind your toupées and your queues, which are things essentially different, and impossible to be mistaken. ”
