Although relay bidding systems can perform well with one person asking questions and making all the decisions, natural bidding systems do better if the auction is a conversation rather than a one-sided decision making process. Too often one person exercises ‘captaincy’ but they don’t have the crucial cards and never find out about them. This hand is one where the final decision is not made by apparent captain. South opens 1S and North invokes a version of Jacoby 2NT where 3C shows all minimums so the 4C bid shows extras and a club shortage. This is a good method because it goes towards refining opener’s range, which many simple response schemes do not, and helps with game or slam decisions. Since South has shown a non-minimum hand (say 14+), North can see 22+hcp in the three suits other than clubs and no club losers. This suggests that slam is, at worst, on one top loser and a finesse but could easily be much better.
South now asks for keycards and finds two plus the queen of trumps and might easily bid 6S as happened at many tables. However, it does no harm to bid 5NT confirming that all keycards are held. Partner will normally bid a side suit king at the six level but is also permitted to jump to 7S with an undisclosed source of tricks. With the good diamond suit, South jumped to the grand slam. This contract would have been precarious without the jack of diamonds but as it is, declarer can easily manage trumps being 2-2 or 3-1 (90%) and diamonds being 3-3 or 4-2 (84%) by ruffing two clubs and making four trumps, five diamonds and the aces of hearts and clubs. Without the jack of diamonds, declarer would need diamonds to be 3-3 and, failing that. The club finesse to be onside which is about 68% and not quite good enough for a grand slam.
Still the lesson here is to have methods whereby both of you can make the final decision.