As South, you open 1NT and partner raises to 3NT. West leads the ♠5. You call for dummy’s ♠9, and East produces the ♠10. Plan the play.
You have some work to do in 3NT, namely, knocking out the ♥A and the ♣A. If the enemy spades are 4–3, you’ll eventually lose two spade tricks as you go about dislodging those aces. If they’re 5–2, however, you need to be a little bit careful at trick one: Duck the ♠10.
Consider the example layout. Say you win the 10 with the ♠Q and, not knowing which defender holds which ace (if West, the defender with the spade length, holds both aces, you’re dead), you decide to go after clubs. East wins the ♣A and plays another spade. If you win that trick, you’ll still need to drive out the ♥A to come to nine tricks. West will win the ♥A and cash his spade winners to beat you.
What if you attack hearts at trick two? That works on the above layout, but if the ♥A and the ♣A are switched in the defenders’ hands, you’ll still go down.
There’s no need to guess which defender has which ace because if you duck the ♠10 at trick one, you’re safe. If East continues spades at trick two to the queen, king and ace, you can drive out whichever ace you please. If East wins, he has no more spades to play. If West, the danger hand, wins and clears the spade suit, you can knock out the other ace.