Signals (signalling) is the term for the defensive play where partners legitimately exchange information about the make-up of their hands.  There are many forms of signals and occasions when they are used, the most common being in response to partners initial lead, when returning partners led suit, and even when following to declarers play in a new suit.  

Discards are also a signal as they may be used to pass information; however, they will be covered seperately.  

The opponents are entitled to know what Signals you have 'agreed' or 'frequently use'.

- Attitude: A 'natural' attitude signal is a high card to encourage continuation or a low card to discourage.

- Count: A 'natural' count signal is a high card (followed by low) to show an even-number and low card (followed by high) to show an odd-number.

- Suit Preference: A 'natural' suit preference signal is a high-card to show some liking for a high suit and low-card for a low suit.  It is easier in a trump contract to be certain of the suit as the preference signal excludes the trump suit.

NOTE: Some partnerships play reverse carding e.g. an attitude signal where a low card is encouraging and a high card discouraging.  This has the advantage that you do not need to play a useful high card to encourage e.g. if you had Q72, is the 7 high or low?  Some believe it is often easier to identify a low card.

The lead of an honour gives partner the opportunity to play any 'free' card (i.e. no need for 3rd man high).  The 'free' card provides the opportunity for an attitude, preference or count signal.

Which signal do you show?

To avoid confusion (and argument) it is best to have some agreement on which signal type to use.  The following is a simple example that attempts to include all 3 signals.

- Attitude on any honour card unless the King is led (K is  Kount).

- Secondary signal. Opinions differ on what to change to when an attitude signal is obvious or not helpful but the expert player usually prefers count as this helps determine distribution except when it is a high level contract (5+).  Decide with your partner what to play for the following:

- Dummy is short in trump contract.  (Preference may allow a suit switch whilst maintaining control of opening suit).

- Dummy has Qxx in suit.  (Preference may allow side suit tricks to be taken before the Q is established for a discard)

- Dummy has AKx and partner leads the Queen.

- King is led and dummy has xxx.  (Attitude when you do not have Ace or Jack in case Declarer has AJx and be trying to get a Vienna coup).

Trump Leads:

Leading a trump can often be a good passive lead (assuming you are not in the habit of leading a singleton trump!!).  It also affords an opportunity to pass a signal with the lead, e.g. a high trump lead can show a liking for the higher non-trump suit whereas a low card denies this.  Partner may also be able to signal in a similar manner.

Returning Suit:

When returning openers lead against a trump contract it is best to play a 'suit preference' signal.  This works well when partner ruffs the return and you want a specific suit played to provide additional ruffing chances.  Against a no-trump contract I prefer to 'complete' the original lead signal i.e. if it was a count signal I play the card that confirms odd or even.

Declarers New Suit:

An opportunity to provide an attitude, count or preference signal exists when declarer opens a new suit.  A common approach is to play preference in a trump contract and count in a no-trump contract.

Recommended Reading:

Title: Better Signalling Now

Author: Mark Horton, 2002, 2014

ISBN: 978-1-55494-616-7  PDF, 978-1-55494-661-7 EPUB

Title: Defensive Signalling

Author: David Bird & Marc Smith, 2001

ISBN: 978-1-55494-258-9