Introduction to Bridge classes typically introduce students to the slogan “eight ever, nine never”. This slogan refers to the decision about whether to finesse for a missing Queen in this sort of position (you have eight combined cards in the suit):
You lead the three to the Ace (both opponents follow); when you lead the Jack from dummy, your right hand opponent (RHO) follows with a small card. Is it better to finesse (hoping for RHO to have the Queen) or to play the Ace (hoping for LHO to have only the Queen remaining)? This situation is the “eight ever” part of the slogan. The slogan suggests, correctly, that when there is no other relevant information, it is better to finesse when you have eight combined cards in the suit. In fact, with eight combined cards and when there is no other relevant information, finessing offers a much better chance to avoid losing a trick in the suit.
But not even the best bridge slogans are infallible. Often, when a thoughtful declarer considers the entire deal, it becomes apparent that although finessing might offer the best chance in this suit, playing the Ace might offer the best chance for a good result on the deal. For more information, see Real World Bridge 3: Declarer Play.