Mind your Ps and Qs

Mind your Ps and Qs is an English expression meaning "mind your manners", "mind your language", "be on your best behaviour" or similar.

Attempts at explaining the origin of the phrase go back to the mid-19th century. One explanation favoured in a letter to the editors of Notes and Queries dated 1851, as well as by the Oxford English Dictionary upon their revision of the relevant entry in 2007, is literal interpretation of the saying, concerning the distinction of the lowercase letters p and q in the context of the school-room or the printing-office. As noted by W. D. Henkle in Educational Notes and Queries in 1876, in this case the proper spelling of the phrase should be "note your p's and q's", because the distinction of majuscule P and Q does not pose a problem.