The Blackwood Four-Five No Trump convention has long been incorporated into our bidding methods as a highly useful device. Unfortunately, however, it has a tendency to break down from overwork. Many experienced players will resort to the Blackwood convention only when the situation cries sharply for its use. Their preference is given to the use of cue bids in many high-level contracts. Less practiced bidders avoid cues for the very sound reason that they cannot be positive their partners would recognize a cue bid if they made one.
Admittedly, there are situations, such as in the hand we are about to discuss, in which even the expert may have trouble with his recognition signals. Usually, however, there is a clue to be discerned from negative inferences—which is a fancy way of saying that you can sometimes tell more from what was not bid than from what was. For a bit of practice in this art, look only at the North cards and consider the bidding.
Was South's bid of five diamonds a cue? What should you do with the North hand? Before you make up your mind, let me fill you in on a few details. East's jump over-call of two spades is pre-emptive rather than strength showing; however, inasmuch as his side is vulnerable, it is to be presumed that he can win about six tricks in his own hand if he had been left at two spades.
Having opened the bidding with the North hand, some players would not venture to give partner a free raise to four hearts. Such inaction would be a mistake because the preemptive bid by East may have put your partner under considerable pressure. If you pass, he may not be able to find another bid; if he does find one (a double, for example), no matter what you decide to do next you are apt to give a wrong impression of your hand.
So, to return to the question before the house, is South's five-diamond call a cue bid or does it show a real suit? Are you permitted to pass and, if so, should you do so?
To answer the first question, you have to consider the bids your partner did not choose to make. He did not cue bid spades at his first opportunity. He did not do so at his last turn, nor did he use a Blackwood four no-trump bid to ask about your aces. The opponents' bidding makes it apparent that South is short in spades; yet he had made no move to give you even tacit support for clubs. Hence, it is virtually certain that he has a two-suiter and that his diamond bid is genuine.
However, since he has given no indication that he is interested in a slam, why did he bother to show the diamonds at all? Since you have already raised hearts, it would be a tactical error on his part to reveal the two-suited nature of his hand to the enemy unless he had a powerful reason for revealing it to you. In all logic, that reason must be that his heart suit is comparatively weak; his diamonds are much saler if the contract is to be played at the five level. Therefore, not only is it permissible for you to pass, but I think it mandatory that you do so with this holding.
That was my reasoning when I held the North cards. As it turned out I had good cause to be happy about my decision to pass. At hearts, there would be no way to avoid three trump losers in addition to a spade trick. Played at diamonds, when that suit divided favorably, South was able to discard all his small hearts on my good clubs and actually ended up making 12 tricks.