Answer: put the five-blank on it

Play the five-blank.

Putting count on a trick your partner is going to win is called smearing. You are handing your own side the points while it is safe to do it.

Here is why it is safe. The double-four is the highest four there is, so nothing that follows fours can beat it. The only thing that can beat it is a trump played by somebody who has no fours. East already played. That leaves West.

Now count the deuces. The double-deuce, deuce-ace, trey-deuce and deuce-blank went in the first trick. The six-deuce and five-deuce went in the third. That is six of the seven. The only deuce you have not seen is the four-deuce. If West has it and has no fours, West can trump this trick. If West does not, North wins it.

Either way, the five-blank is not going to do better later. It is the lowest five there is, and it is not a trump, so it is not likely to win a trick on its own. If you keep it, it will land on some later trick, and that trick could just as easily go to the other side.

We checked this with a small program. It tried every way the dominoes you cannot see could be split up, 90 of them that fit what has been played, and played the rest of the hand out the best way for both sides each time. In none of them did keeping the five-blank do better than playing it now.

In the real hand, West did hold the four-deuce. But West also held one plain four, the four-ace, and had to follow suit with it. A player who can follow suit is not allowed to trump. So North's trick picked up 1 point for the trick, 5 for your five-blank and 5 for West's four-ace. That is 11 points. Throw the ace-blank there instead and the trick is only worth 6, and your nickel ends up on a trick East and West win.

The habit: when partner's domino cannot be beaten, or cannot be beaten by anything you would save the nickel from, put the count on it.