Third Hand High

This is perhaps the single most important motto for defenders. Bridge is the ultimate partnership game. When you are playing the third card to the trick, your partner has already played a card. If your partner has led low and the second card (of dummy’s, or declarer’s if during the play) is also low, you must play high, or declarer will win a cheap trick.
 

After ♠3, ♠2, you must play ♠Q, or declarer will win a cheap trick with ♠J.

After ♥3, ♥2, you must play ♥A, not ♥J, to prevent declarer winning ♥Q. After winning ♥A, you should return ♥J, the top of two remaining. In this way the defence can scoop the entire suit without blockage (especially important at notrumps).