A Subtle Promotion

In notrump, each side can run at least seven top tricks. East/West can take 10 tricks by leading a diamond to the ace, club finesse, run four diamonds, then finesse clubs again and run clubs. North/South get seven tricks by leading a spade to the king, heart finesse, heart king, another spade, and then running three good hearts.

Against a spade contract, East/West must get a spade, so they simply take their six top tricks in the minors.

Against a heart contract, East/West start the same way - diamond, club finesse, two diamonds, club finesse, club ace, then a fourth club. East ruffs high with the queen (or 10) and this promotes West's ♥ 9.

If East/West declare in diamonds, the defense takes two top spades, finesses in hearts, cashes the ♥ K, ruffs a spade, cashes the ♥ A, and finishes up with a fourth round of hearts, promoting South's ♦ J.

The trickiest case is when East/West declare clubs.

North/South take their two spades, three hearts, and a spade ruff as before. On the third round of hearts, South pitches a diamond, and, when North leads a fourth round of hearts, South pitches a diamond again at this position:

What should East/West do? If they ruff in East, they can only finesse once in trumps. But if East pitches a diamond and ruff with the West hand, they only have one entry to dummy, since South can ruff a second diamond entry, so again they can only finesse in clubs once.