Standard Blackwood
Where standard Blackwood 4NT is in force, a four notrump bid (4NT) asks partner to disclose the number of aces in his hand. With no aces or four, partner replies 5♣; with one, two, or three aces, 5♦, 5♥, or 5♠, respectively. The difference between no aces and four is clear to the Blackwood bidder (unless the partnership lacks all four) so one member of the partnership knows the combined number of aces. That is often sufficient to set the final contract.
The continuation bid of 5NT asks for the number of kings according to the same code of replies at the six-level: 6♣ shows no kings or four, etc. Asking for the number of kings confirms that the partnership holds all four aces, so partner may reply at the seven level with expectation of taking thirteen tricks. (A common agreement is that when spades is not the trump suit, 5♠ asks respondent to bid 5NT. That is useful when the reply to 4NT bypasses the intended trump suit but also shows that slam is likely to be a poor contract because two aces are missing.)
A void may be as good as an ace in some situations but it should not be counted as an ace. Some experts (Kantar for one) recommend the 5NT reply to 4NT – the cheapest with no standardly assigned meaning – to show a void plus two aces and six of a suit to show a void in the bid suit plus one ace.
Read more about this topic: Blackwood Convention
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