An obligatory finesse occurs when declarer is "obliged" to duck an honor. For example:
Needing 3 tricks in the suit, you lead toward the king, which holds the trick. You continue with a low card from dummy, East playing low. At this point, you have an obligatory finesse. Play low from hand and hope that West started life with the doubleton ace.
A similar example needing 4 tricks:
You lead low to the queen, which wins. A spade back to hand should be ducked in case West began with the doubleton ace.
One last example. This following deal comes from Ely Culbertson's "Contract Bridge Complete." South plays in 6.
| ||North|| |
"West opens the deuce of diamonds (obviously a singleton), dummy winning with the Ace. The Ace and King of hearts are led and when West shows out, East is marked with ten red and (consequently) three black cards. Trumps must be drawn, so the heart Queen is cashed and declarer enters his hand with the Ace of spades to lead the Jack of hearts, dummy discarding a low diamond. The two black Kings are led and when East follows to both, the location of every card is marked. The low club is led and the ten is finessed. The Jack of spades is returned, South discarding the Ace of clubs. West wins with the Queen and must yield entry to dummy with a spade or club, affording discards for South's losing diamonds."