_Mind your P's and Q's_ (Vol. iii., pp. 328. 357. 463.).--When I
proposed this Query, I mentioned that I had heard one derivation of the
phrase. As it is different from either of those which have been sent,
it may, perhaps, be worth insertion. I was told by a printer that the
phrase had originated among those of his craft, since young compositors
experience great difficulty in discriminating between the types of the
two letters.