During the auction you can ask a question whenever it is your turn to bid.

However you should NOT ask a question at that stage if the answer can have no effect on what you intend to do. If you ask a question and then pass you put your partner in a difficult position. He may have a marginal decision as whether to bid, double or pass and the fact that you have asked a question should indicate you have some interest in taking part in the auction or have some values in your hand. If partner now bids your opponents may think he was influenced by your question and complain. Note you cannot ask a question when it is your partner’s turn to bid.

When the bidding has ended and you are on lead you can ask any questions you like before selecting your opening lead. Your partner must remain silent while you do so to avoid any suggestion that your subsequent choice of a opening lead is affected by a question or comment made by him.

If you are not on lead you must wait until your partner has selected his opening lead and placed it face down on the table before you ask any questions. This is why the bidding cards are not removed until all questions have been asked and the opening lead is turned over. Both defenders can see the whole sequence of bids at the time they ask questions.