West   North   East   South
                                 4♥
all pass

Partner leads the ♦2 against 4♥, dummy's jack wins the trick. Declarer ruffs a club in hand at trick two, partner following with the ♣9, and leads the ♥8, partner plays low, you win the trick with the king. What do you play now?

Solution:
It is possible that partner has led a singleton diamond, in which case you should give him a diamond ruff. However, partner's play of the ♣9 should carry significance - it is the highest club he could spare. Partner is showing interest in spades, and you should play a low spade to partner's king, win the spade return with the ace, and give partner a spade ruff to defeat the contract.

If declarer wins the opening diamond lead in hand and plays the ♥8, then your task becomes truly difficult, as you don't have access to a signal from partner in clubs. Even in that scenario, partner's card in the trump suit should be a suit-preference signal.

Bridge Baron's Double Dummy Analysis:
Bridge Baron's double dummy analysis confirms that apart from an opening ♠K lead, a low spade at trick four is required to defeat the contract.
Bridge Baron's Par Contract Analysis:
The par contract on this deal is 4♠ by North-South.

Bridge Baron 16 deal No :