Source: http://www.michaelslawrence.com/
There are hundreds of little questions that you must know the answers to if you and your partner will be good defenders. Should you lead, for instance, the king or the ace from the AKx? Are fourth best leads best? What does your partnership lead from the KJ103? You have to know these things. The hand here shows yet another area that needs clarification.
There is an old argument about what card you should lead from various holdings in a suit your partner has bid. Some say that you should lead the three from J93 and some say that you should lead the jack. Do you and your partner know for sure which card you would lead from the J93 when your partner opens the bidding in this suit? Does it make any difference if your suit is headed by the ten or nine or lower?
Both Sides Vulnerable
East Deals
Look at the North and East hands. East bids a few times and pushes South to five spades. West leads the ten of clubs.
East takes his jack and South plays the five. When East cashes the ace of clubs, declarer plays the seven and West plays the eight. The three of clubs is missing. Should East try to cash a third club trick or should he lead a high diamond? Or do you think it makes no difference?
Lead: 
10
DO YOU HAVE AN UNDERSTANDING?
Having good understandings will give you the answer. Does your partnership know what the partnership lead is when you have three cards in a suit?
Does it make any difference to you if your partner has bid the suit?
Does it make any difference to you if you have supported this suit?
Current thinking dictates that you lead a low card when you have three or four cards to an honor in partner’s suit. (Count the ten as an honor.) If East West agree with this rule, East will play West for only two clubs and will cash the king of clubs. As you can see from the layout, it is necessary to defend this way.
This agreement is a good one to have. There are many many cases where leading the honor card will cost a trick. Even if it doesn’t cost a trick, your partner may be confused about how many cards you have in the suit. Adopting this lead rule will help in many ways.
A second, but equally important question, is what West should lead if his clubs are the 753. If he is going to lead this suit, should he lead the three, the five, or the seven?
In advance of this discussion, I will tell you that leading the five is awful. Do not lead the middle card when you have three of them. Your partner won’t be able to read your lead and he may try to give you a ruff or he may play you for an honor. Why give your partner two ways to go wrong?
My choice is this.
If I have not supported the suit, I lead the small card. My partner may err by thinking I have an honor in the suit. This is not nice, but at least my partner won’t think I am ruffing the third round.
If I HAVE supported the suit, then I lead the top card. Because I raised, my partner will know I don’t have a doubleton, and by leading a high card, he will use the rule of eleven and determine that I don’t have an honor.
This is not a perfect method, but because it is consistent, your partner will do the right thing more often than when you have no method at all.
Let me finish with this observation. Leading from three small cards is a lousy way to start a defense. When the best choice you have is to lead from three little cards, you are not having a good day.
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