Defensive Signalling (Might be more beginner than intermediate, but check my work, please)
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Standard opening lead signals are marked in bold on a pre-printed ACBL convention card:

1. Top of an honor sequence (Q from QJT(xx), K from KQ(xx) against suits, etc.).

2. Top of an interior sequence (J from AJT(xx), T from KT9(xx), etc.).

3. 4th best spot card from length (xxxxx, KJxx, etc.). Partner of opening leader (and declarer) can apply Rule of 11 to determine how many cards higher than the led card are outside of the opening leader's hand. As an example, the Opening lead is the 4 of Diamonds. There are 7 Diamonds higher than the 4 that are not held by the opening leader. You look at your own hand and dummy to determine how many of those are in the hidden hand.

4. Low from 3 to an honor (Qxx, Kxx,etc.). Note that if the lead is from 3 to an honor, players should be applying a Rule of 12 (but won't know that they should be).

5. High Low from a doubleton (xx).

6. Strictly standard is low from 3 small (xxx) against suits but a significant proportion of players play either high (attitude) or MUD (middle, up, down).

7. It is standard enough to lead a high-ish card from a suit of xxxx (or longer) that we don't want partner to return, but it is worth discussing with a partner that says "standard carding."

We have modified this by explicit agreement in two significant ways:

1. We lead low from 2 small (xx) against suits only and high from 3 small (xxx). We also lead 2nd high from 4 small cards. This is called "2nd and 4th." Generally a low card indicates that we want partner to return the suit and a high card indicates that partner should look elsewhere for strength in our hand. This lead of low from strength and high from nothing, allows partner to confidently make finessing plays such as your play of the 9 from Q9x on the opening lead of the 2 of Spades on Board 15 from FR 11/2/18. If our leading convention calls for the lead of a high spot card and our high card is a 9 or T, we may lead second high to avoid giving up a trick with our opening lead signal.

2. When leading from an honor sequence, we use an asking code rather than a showing code. The lead of the Ace or the Queen asks for an attitude signal. In the case of an Ace, we signal positive attitude (low to high) with the Q or better or if we have a doubleton. Otherwise we signal discouragement (high to low). In the case of a Queen, we signal positive attitude with the J or better. I do not think that we should encourage with a doubleton on the lead of the Q but we can discuss. The lead of the King asks for a count signal.

Usually the opening leader will ask for attitude when leading from a shorter suit (AKx) or if there is some urgent need to get partner on lead -- generally to shift to a different suit. With a longer suit, four cards in length or longer, opening leader usually wants to know if a second trick can be cashed and so will lead the K to get a count signal. Looking at dummy and her own hand, and adding the length shown by partner's count signal, Opening leader can deduce how many cards are held by declarer and most importantly, whether the second high honor will cash or be ruffed.

If OPPonents ask about our honor leads, I respond by saying "Honor leads ask. They don't show. The lead of the Ace, the King or the Queen may be made from any holding AK(xx), KQ(xx), QJ(xx)." As you get more comfortable with defense, you will know from your own hand and the bidding what kind of signal you want to request from partner in various circumstances.

In addition, a fair sized minority of players, adopt a convention where the lead of the J denies a higher honor and the lead of a T or a 9 shows "zero or two" higher. We do not play this and for us the lead of a J (against NT) could be from AJTxx (Board 23 from FR 11/2/18). This convention is variously called "Jack denies" or "zero or two" or "coded 9s and Ts" (typically depending on whether the lead is the J or a T or 9).

Next up Third Hand play.

In standard carding, playing high to low shows either that we like (we hold a high honor in) the suit led if it is an attitude signal or that we hold an even number of cards in the suit led if it is a count signal. Conversely, playing low to high in standard carding shows dislike (holding no high honor) for the suit led if showing attitude or an odd number of card in the suit led if showing count.

Overriding any signalling option is the third hand's obligation to play "third hand high" on the opening lead of a spot card to help set up any honor(s) in the opening leader's hand. Thus, from a holding of Q84 and an opening lead of the 3, everyone plays 3rd hand high, or the Q on the assumption that partner is leading from the K, the J or from both (usually not from the A, but possible against NT).

If 3rd hand wins trick 1 by playing 3rd hand high and decides to continue the suit led, her lead to trick 2 is typically a count signal. From an original holding of 2 cards in the suit led, 3rd hand must (if continuing the suit) play her remaining card. From an original holding of 3 cards in the suit led, 3rd hand continues high to low, with the original middle card from the holding. From an original holding of four or more cards in the suit led, 3rd hand continues with her original 4th best. Winning the K in the suit lead and returning the 2 shows a holding of 2 or 4. Winning the K and returning the 9 shows an original holding of 3 (or 2).

Usually after the play to the first two tricks when 3rd hand wins trick 1, both defenders have a fairly good idea of the holdings in the unseen hands (or even a complete count of this suit) in this suit. The suit led is often a critical suit for the defense and these signals clarify the position in that critical suit.

If 3rd hand cannot beat the card played by dummy, it is traditional but by no means universal, to give a count signal to partner in standard carding. Also, if dummy holds one or more honor(s) that declarer chooses not to play, third hand may insert an intermediate card (J, T, 9 or even 8) choosing to hold a higher honor over the card retained in dummy. These "trick preserving" plays, take priority over any signalling obligation.

If you are unsure what to signal, or what partner's signal is to trick 1, the default agreement/convention is attitude. Attitude to partner's leads unless clearly and explicitly agreed otherwise is a fine approach and will avoid problems later in the hand and especially in the post mortem.

We have two explicit agreements that modify the above. Even so, I want you to be aware of what is "standard" because you will from time to time have partners other than me.

1. UDCA

We have agreed to play upside down count and attitude (UDCA). If we like a lead, we signal that with a low card. If we have an even number of cards we also signal that with a low card. The theory behind UDCA is that we save our higher cards in our stronger suits and use higher cards in weaker suits for signalling.