Blackwood Convention: Counting Aces for Slam
Blackwood is the best-known slam convention in bridge. A single bid of 4NT asks partner how many aces they hold, so you never bid a slam off two cashing aces. It is simple, powerful and on nearly every convention card.
What Blackwood Asks
Slams score huge bonuses, but only if your side is not missing two top tricks. Blackwood is the safety check: after a trump fit is agreed and one partner senses slam, a bid of 4NT asks “how many aces do you have?”
It is artificial — 4NT here is not a natural offer to play no-trump. The reply lets you count the side’s aces and avoid the catastrophe of bidding six with two off the top. See how it fits the bigger picture in our slam bidding guide.
Responses to 4NT (Standard Blackwood)
Partner answers up the ladder, one step per ace. The scale is fixed and easy to memorise — clubs, diamonds, hearts, spades for zero-or-four, one, two and three.
Showing your aces
A Worked Example — Blackwood in Action
Partner has just raised your 1♠ opening to 3♠, a limit raise promising four trumps and about 10–12 points. You pick your hand back up:
Spades are agreed and you hold a monster — 20 points and three aces. Slam is clearly live, but jumping to 6♠ could be a disaster if partner is missing the two aces you do not hold. So you bid 4NT.
Partner replies 5♦, showing one ace. That completes the set: your three plus partner’s one means all four aces are present and the opponents cannot cash two off the top. You bid 6♠ with confidence. Had partner shown zero aces with 5♣, you would have signed off in 5♠ and lived.
After the Reply — Signing Off or Driving On
Blackwood only counts aces; you still have to make the decision. Two follow-ups cover almost every situation.
Signing off below slam
If partner’s answer reveals the side is missing two aces, do not bid the slam. Return to the agreed trump suit at the five level — for example, 5♠ over a 5♦ reply — and partner must pass. The whole point of asking was to be able to stop.
The 5NT king-ask
When every ace is accounted for and a grand slam is in view, bid 5NT to ask for kings. Partner answers on the same scale — 6♣ for zero or four kings, 6♦ for one, and so on. Bidding 5NT also guarantees the partnership holds all four aces, so it doubles as a grand-slam try.
When Not to Use Blackwood
✗ Blackwood is the wrong tool when
- You hold a void — an ace-count alone can mislead you
- You have a weak doubleton in a side suit (you may be off the A-K there)
- You only want to be in game — never start an ace ask without slam values
♠ Consider instead
- Roman Key Card Blackwood — counts the trump king and queen too
- Gerber (4♣) — the ace-ask after a no-trump opening
- Control-bidding (cue bids) when shape matters more than raw aces
Common Blackwood Mistakes to Avoid
- Bidding 4NT with no trump fit agreed. Without an agreed suit, 4NT is natural and quantitative — not Blackwood. Set the trump suit first.
- Asking when you cannot use the answer. If you will bid the slam regardless, or have no slam values at all, asking only tips off the defence.
- Using Blackwood with a void. Ace-counting breaks down opposite shortage — switch to RKCB or cue-bidding.
- Forgetting the 5♣ ambiguity. Always work out from the prior bidding whether 5♣ is zero or four before you act on it.
Key Takeaways
- 4NT asks for aces — but only once a trump fit is agreed.
- Replies climb the ladder: 5♣ (0/4), 5♦ (1), 5♥ (2), 5♠ (3).
- Missing two aces? Sign off in five of the trump suit.
- 5NT asks for kings and promises all four aces — a grand-slam try.
- With a void, prefer Roman Key Card Blackwood or cue-bidding.
Related Guides
Frequently Asked Questions
The convention was devised by American player Easley Blackwood in 1933 and has been a fixture of slam bidding ever since.
No. 4NT is Blackwood only after a suit fit is agreed. A direct 4NT raise of a natural no-trump bid is usually quantitative — inviting 6NT — not an ace ask.
Both ask for aces, but Gerber uses 4 clubs and is reserved for auctions that begin with a no-trump bid, where 4NT would be needed as a natural raise.
5 clubs shows either zero or four aces. The earlier bidding almost always makes clear which it is: a partner who has limited a weak hand has none, while a strong hand has all four.
Learn standard Blackwood first — it is simpler. Move to Roman Key Card Blackwood once counting aces feels automatic, because it adds the vital trump king and queen.
The common agreement is DOPI: a double shows 0 aces, pass shows 1, and the next two steps show 2 and 3. Agree it with your partner before you need it.