Its purpose is to enable the partnership to explore its possession of aces, kings and in some variants, the queen of trumps to judge whether a slam would be a feasible contract.
After developing the concept in 1933, Easley Blackwood submitted an article proposing his slam-seeking convention to The Bridge World magazine but it was rejected.
"[2] He pointed out several misconceptions and concluded with a fifteen-point summary of the "complete and official" Blackwood Slam Convention.
In some situations where 4NT is a quantitative invitation, especially where 4♣ is a jump, many partnerships use the Gerber convention instead of the Blackwood family: 4♣ asks for the number of aces or key cards.
Where standard Blackwood 4NT is in force, a four notrump bid (4NT) asks partner to disclose the number of aces in his hand.
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.)
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.
The basic outline of responses is: In practice, the ambiguity is unlikely to occur, as a strength difference between hands with 0 or 1 and 3 or 4 aces is big enough that it can be established in previous rounds of bidding.
In other words, a partner who has previously shown, for example, 12-15 range of high points is unlikely to hold 3 aces for his bid, etc.
If the querying partner ascertains that all aces are present, he can continue as follows: Roman Key Card Blackwood (RKCB) has largely replaced the standard version among tournament players.
Evidence for that inference includes the entire auction as well as the number of key cards that the 4NT bidder holds.
(Responder may also show the queen with extra length in trumps, where the ace and king will probably draw all outstanding cards in the suit.)
The 5♣ and 5♦ replies tell nothing about the queen or extra length, but the 4NT bidder may ask about that using the cheapest bid other than five of the trump suit.
"Minorwood" is a variation of Blackwood, in which the minor suit which the partners agree will be trumps is itself used as the ace/key card ask.
Many players, even experts, refuse to play Exclusion Blackwood because of the potential disaster of forgetting the agreement.