Here is a brief summary of 3rd & 5th opening leads:
- This lead convention only applies to the opening lead against a suit (not no-trump) contract. (Against no-trump contracts, it remains best to lead 4th best)
- If you decide to lead a low card against a suit contract in a suit partner has not bid …
- From a 3 card suit, lead your lowest (3rd best);
- From a 4 card suit, lead your 2nd lowest (3rd best)
- From a 5 card suit, lead your lowest (5th best)
- (From a doubleton, lead the top card, as usual)
- (From a 6 card suit [very rare], there is a split of opinion — most would advocate leading the 5th best)
- 3rd hand interprets the lead as follows:
- If 3rd hand can tell (upon inspecting the opening lead, the cards in dummy’s and his own hand and the card played by declarer on the 1st trick) that partner has led his lowest card in the suit, then partner has led from a 1, 3 or 5 card suit.
- If, after partner’s opening lead, there is a low card still missing, then 3rd hand deduces that partner may have that card — so partner has led from a 2, 4 (or rarely 6) card suit.
- It is typically the case, based upon the bidding and other easily available inferences, that 3rd hand can then figure out the exact number of cards partner originally held in the suit.
- Note that this opening lead tells 3rd hand nothing about partner’s honor strength in the suit led: it only gives his/her count in the suit — either even or odd. Holding either K 3 2 or 4 3 2 in a suit, partner would lead the 2 in both cases (3rd best). The exception to this is if partner has supported a suit you have bid: in that case, you know that partner has at least 3 cards in your suit, so, with a 3 spot cards in your suit, she can lead the highest spot card to show she does not have an honor; with 3 card support including an honor, she can lead her lowest card suggesting she had an honor, without misleading you about her count.