What is the origin of the phrase, 'Mind your Ps and Qs'?

P Wilden, Norwich

• "P"leases and Than"Q"s.

  Simon, London

• Isn't this to do with the two branches of Celtic languages? The 'P' branch (Welsh, Cornish and Breton - all derived from the old British or Brythonic language) has words such as 'pen' (Welsh for head or end) starting with the letter p, while languages in the 'Q' branch (Scottish, Irish and Manx) start similar common words with something approximating to the 'q' sound (they actually use the letter 'c', usually followed by a vowel). Thus the Scottish and Irish equivalent of Welsh 'pen' is 'ceann'.

  Steve Lake, Cardiff