Blackwood Convention: 4NT Asks for Aces
Once you and partner agree you belong in slam, the danger is bidding one off two missing aces. Blackwood is the safety check — a single 4NT bid that counts the aces before you commit to the twelve-trick level.
Why Blackwood Exists
A small slam asks you to win twelve of thirteen tricks. You can have all the high-card points in the world, but if the opponents hold two aces they simply cash them and you are one off before you start. Counting points tells you about strength; Blackwood tells you about controls — the aces and kings that stop the defense taking quick tricks.
The convention, devised by Easley Blackwood in 1933, is beautifully simple: once a trump suit is agreed and the partnership smells slam, a bid of 4NT asks "how many aces do you have?"
The Responses to 4NT
Partner answers in steps, starting with the cheapest bid. In standard Blackwood the replies count aces directly.
Standard Blackwood Responses
Add the reply to your own aces. If the partnership is missing two aces, sign off in five of the trump suit. If only one (or none) is missing, you are safe to bid the slam.
Asking for Kings with 5NT
If after the ace reply you are interested in a grand slam — all thirteen tricks — a continuation of 5NT asks for kings, with the same stepwise answers (6♣ = 0 or 4, 6♦ = 1, and so on). Bidding 5NT also confirms the partnership holds all four aces, since you would never explore a grand slam with one missing.
A Worked Example
Spades are agreed and you, holding the strong hand below, are weighing a slam.
You hold three aces yourself. Slam looks close, but a grand slam needs the last ace and the diamond control. Bid 4NT: if partner shows one ace (5♦), the partnership has all four and you can investigate 5NT for kings; if partner shows none (5♣), settle for the small slam, because a grand could be off the missing ace.
Blackwood, RKCB and Gerber
Standard Blackwood counts aces only. Most tournament players upgrade to Roman Key Card Blackwood, which treats the trump king as a fifth "key card" and also reveals the trump queen — far more precise for slam decisions. When the agreed suit is clubs, 4NT is too high to be safe, so partnerships use Gerber (4♣) to ask for aces instead.
Common Blackwood Mistakes
- Bidding 4NT with no agreed trump suit. Without a fit, 4NT is natural and quantitative — not Blackwood.
- Using Blackwood with a worthless doubleton. Even with every ace, a side suit of two small cards can leak two fast losers. Count losers, not just aces.
- Forgetting whose turn it is to sign off. The 4NT bidder is captain and decides the final contract after hearing the reply.
- Asking when you can already count the slam. If you know slam is safe, just bid it — don’t give the opponents a free round of information.
Key Takeaways
- 4NT asks for aces once a trump fit and slam values are agreed.
- Replies: 5♣ = 0 or 4, 5♦ = 1, 5♥ = 2, 5♠ = 3.
- Missing two aces → sign off; missing one or none → bid the slam.
- 5NT follows up to ask for kings and confirms all aces are held.
- Upgrade to RKCB for precision, and use Gerber when clubs are trumps.
Related Guides
Frequently Asked Questions
Once a trump suit is agreed, 4NT is the Blackwood ace-ask. It asks partner to show how many aces they hold before the partnership commits to a slam.
5 clubs shows 0 or 4 aces, 5 diamonds shows 1, 5 hearts shows 2, and 5 spades shows 3. The auction makes clear whether 5 clubs means none or all four.
After the ace response, bid 5NT to ask for kings. The replies follow the same steps one level higher, and 5NT also confirms the partnership holds all four aces.
Yes. Blackwood is a slam tool. You should only use it once you have agreed a trump fit and decided you want to be in slam unless too many aces are missing.
Avoid it with a worthless doubleton or a void, where counting aces can mislead you, and never use it without an agreed trump suit, since 4NT is then natural.
Both ask for aces. Blackwood uses 4NT and suits other agreed suits, while Gerber uses 4 clubs and is used mainly after no-trump bidding where 4NT would be too high.