A bridge bidding convention used in determining whether a partnership is ready to bid a slam by asking for aces.

Except in certain situations where 4NT is a natural bid, it is an artificial bid initiating Blackwood. In the most basic original form of Blackwood, the partner of 4NT bidder responds with 5C if he holds zero or four aces, 5D for one ace, 5H for two, and 5S for three. The 5C bid is not ambiguous because it is generally impossible for a partnership to be considering slam if they lack all four aces, and in the weird exceptions, 4NT bidder knows whether to expect a strong hand (that could have all 4 aces) with pa rtner or not.

Some people play variations such as Roman Key-Card Blackwood in which the King of the agreed-upon trump suit counts as a fifth ace, and the 5D response indicates one or five key cards.

In any case, the 4NT bidder may continue Blackwood by bidding 5NT, which guarantees he holds all the aces or key cards that 4NT was asking for, and asks for kings with the same scheme. If the king of trump was counted in the 4NT bidding, it doesn't count again; usually the queen of trump counts as a king in its place.

Gerber is a similar convention used to ask for aces on a no-trump hand.