The method of conveying information between the defenders. You can do this both when following to a suit and when discarding. Common types of signal are attitude signals – when you indicate whether you like a suit or not – length signals – when you show whether you have an odd or even number of cards in the suit – and suit-preference signals – when you show a preference for some other suit. Defensive signals are vital for defending accurately.
For signals to work it is important not only that you give the right signal but also that partner is watching – and vice versa when partner gives a signal.
Here is an example where you can give all three of the common types of signal on the same deal:
South plays in 3NT without having bid any suits. When West leads the ♠Q, you, East, follow with the ♠4, your lowest card in the suit, as an attitude signal to say that you do not like spades.
Declarer plays a club to the queen and you follow with the ♣2, your lowest card in that suit, as a count signal to show an odd number of clubs. On the second round of clubs, having already given count, you play the ♣8, the higher of your remaining clubs, as a suit-preference signal for the higher red suit.