The BLACKWOOD CONVENTION

The Blackwood Convention

Bidding Conventions are predetermined coded messages in the auction (using regular bids) to convey special features of a hand (Bidding Guide p.14).
The Blackwood Convention starts with a bid of :

4 NT which conveys the message :
"Partner, how many Aces do you hold in your hand?"

Exception
In "1NT - 4 NT" , and "2NT - 4NT" the 4NT is not Blackwood but a natural invitation to Small Slam.

Partner replies :

5 C - with 0 or 4 Aces

5 D - with 1 Ace

5 H - with 2 Aces

5 S - with 3 Aces

These bids tell absolutely nothing about the bid suit, but only indicate the number of Aces held in Partner's hand.
A follow up bid of :

5 NT shows that the side holds all 4 Aces and conveys the message :
"Partner, how many Kings do you hold in your hand?"

Partner replies :

6 C - with no King

6 D - with 1 King

6 H - with 2 Kings

6 S - with 3 Kings

6 NT - with 4 Kings

The Blackwood convention can be useful in trump contracts where some of the 33 points required for a Small Slam may include 4 or more shortages points, and the opponents may therefore hold 2 Aces. The Blackwood Convention can guard against this.

Many players, especially beginners, go into Blackwood as soon as they sniff a Slam possibility. This is usually totally unnecessary. Checking up on Aces and Kings is entirely a precautionary measure. In most cases it is more useful to the opponents, and it will never create more points!