Possibly the most popular convention of them all. This is the use of, 4NT and 5NT, after a trump suit has been established, to ask for the number of aces and kings held. The responses are:

5C shows one zero or four aces
5D shows one ace
5H shows two aces
5S shows three aces

If the 4NT bidder continues with 5NT, the responses are the same, but one level higher, and are for the kings. If the Blackwood bidder decides the best contract would be 5NT, the bidder must make an unusual suit bid after the response to 4NT to request the partner to convert the bid to 5NT.

See also Defence to Blackwood Interference.

When is the bid not Blackwood?

  1. if partners last bid was 1NT, 2NT, or 3NT, or if your last bid was 3NT;
  2. immediately after Stayman, Jacoby transfer, or minor suit transfer bid;
  3. if partner has never bid or doubled.

In all other situations 4NT is Blackwood.