Defensive Carding.  

Basically, low cards are discouraging and high cards 
are encouraging.  The majority of shifts are obvious 
if we are both counting and working on the same page.  
This will take a while to build confidence and 
understanding, but if you know that I am always trying 
to play the right card and I know that you are doing 
the same, it will work itself out.  Both partners 
should be aware that they won't always have the card 
that they want to have for signaling.

When you are defending, every card you play sends a 
message to your partner.  Send the right message, play 
the right card.

Try to play the card that will give your partner the 
most information possible while not giving too much 
information to the declarer.  For instance, if a card 
is led from the dummy and you hold the JT9 of that suit, 
play the nine.  This denies the eight but does not deny 
the ten.  On the other hand, when you are on lead as 
defender, from the holding of JT9 you should lead the 
jack - this denies the queen and implies the ten - you 
may not always have the ten, but in most cases, you will.  
Eventually, by being consistent, you will assist your 
partner in determining what cards you hold and from that,
what the declarer holds.

When partner makes the opening lead, or for that matter,
any lead, it is your job to play the card that shows 
either attitude, count, or suit preference.  The 
situation and the cards in the dummy will help you to 
determine how to signal.  Let's assume that partner 
leads the king of a non-trump suit and the dummy has 
three small cards in that suit.  You should play a 
card that reflects your attitude with regard to the
continuation of the suit.  If you hold the ace or the 
jack, play an encouraging card, otherwise, a discouraging
card.  This, of course, depends on your having a card 
that will send that message.

Now let's assume that the ace-third of the suit led is
in the dummy.  Partner must have the queen to have led the
king, so your card will depend on what is played from the 
dummy.  If dummy plays low, encourage if you hold the jack
but discourage if you do not.  If dummy wins the ace, you card 
should be a "count" card.  Your lowest card in the suit 
would indicate an odd number of cards (1,3, or 5), while a
higher card would indicate an even number.

Take this concept one step farther.  Assume that the dummy 
has a singleton in the suit led.  Take this opportunity to 
make a "suit preference" play.  A higher card would indicate
a preference for the higher ranking non-trump suit, while a 
lower card would show preference for the lower ranking
non-trump suit.

So, the hierarchy of carding is Attitude, Count, and then, 
Suit Preference.  Like everything else in this game, knowing
when each of this situations is called for, and being on the
same page as your partner, is the key to success.  Remember,
you cannot give two signals with the same card.  Don't even 
try! Figure out what is appropriate and play that card.  And
be forgiving!  Partner is trying to play the right card but
may not always have the card that would be easiest to figure
out.  High and Low are relative terms.  A six could be either
depending on the other cards you can see in that suit.  If, 
between your hand and the dummy, you cannot see any of the 
cards lower than the six, than partner is probably playing 
a high card.  On the other hand, if you can see all of the
cards lower than the six, partner is playing a low card.

Discarding is usually a suit preference situation.  Normally, 
the first chance to discard occurs when the declarer is pulling
trump.  Your card should have some meaning to your partner, and
what meaning it has depends on your agreements.  Here are some
of your options.

Standard discards.  A higher card indicates preference for the
suit being dicarded.  A lower card indicates a dislike for the
suit beind discarded.

Upside down attitude.  A lower card indicates preference for the
suit being dicarded.  A higher card indicates a dislike for the
suit beind discarded.

Lavinthal discards.  At the first opportunity to discard, a card 
of a suit not liked will be discarded.  The rank of that card will
indicate a preference for the higher or lower of the other two 
non-trump suits.

As you can see, it would be good, as declarer, to know what the
agreements of the defenders are.  Don't be afraid to ask.  You may
look at the convention card of the opponents or simply turn to the
partner of the player making the discard and ask "What are your 
carding agreements?"
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