Yee-Ming, apologies ... Clearly the American imperialist double quotation mark has cast its net further than I thought. Thank goodness the UK never had such imperial ambitions ... In British publishing, single quotation marks should always be used when quotation marks are needed. The only exception is when you have quotation marks within a set of quotation marks (e.g. somebody is being quoted quoting somebody else), when the double quotation marks are used for the 'internal' quotation. The USA and other countries have different rules on this. There is no 'right' or 'wrong' usage (or "right" or "wrong" for that matter).