This is an arbitrary finesse, which is often forced upon you, when,
unless the cards of a suit are in a certain position, there is no
possibility of winning. Perhaps the best way of defining the finesse
obligatory is to give an example showing the position as it most
frequently occurs:
K♠ 7♠ 3♠
x-------x
| Y |
| A B |
| Z |
x-------x
Q♠ 9♠ 8♠ 6♠ 5♠
“Z” is the dealer, and leads a low card to the king in dummy, which
wins the first trick. The suit is returned, the ace being marked with
“A.” If “A” holds both ace and knave, no matter how the dealer plays,
“A” must win two tricks. Unless the knave is in “B’s” hand, and “A”
holds the singleton ace, the adversary must win two tricks. The
dealer should, therefore, play his lowest card, not the queen.