Over the years there I has been a tug of war between 4th best leads vrs 3rd and 5th. Experts have argued which lead convention is better. The latest consensus among experts at the national level is that they both are ! 3rd & 5th best lead are better against suit contracts & 4th best against NT contracts. In suit contracts , it is important to know that partners low card lead is 3rd or 5th for cash out purposes , switches & planning your defense. It is less important in suit contracts that you “burn spot cards” by leading your 2nd lowest card to show 4 of the suit.
In NT, when you waste spot cards on your opening lead with 4 of them , it can often be fatal. Also the rule of 11 in NT contracts is invaluable in knowing what is going on with the suit. Bryan Maksysmetz knew that I played 4th best with Tom & 3rd & 5th with some partners. Therefore , he suggested that we should be a switch hitter in this regard. He says most top level players alternate between suit & NT contracts with 3rd and 5th best leads. I can see the advantages now that I play both lead conventions with my two regular partners.
Playing 3rd & 5th best with some partners , has opened my eyes on the importance of differentiating between 3 to an honour lead as opposed to 4 of an honour in suit contracts. With standard 4th best , you cannot tell the difference which can be fatal in suit contracts. In NT contracts , 3 card suits are hardly ever good opening leads except when demanded by the bidding , so partner can normally work it out anyway from the distribution given by the bidding.. Ambiguity is the killer in Bridge & 3rd & 5th solves one of the worst guesses in Bridge.
Most experts play “present count” instead of original count. When you are returning a suit in NT as opposed to a suit , you are playing 4th best ( present count) . In a suit contract , you are returning 3rd & 5th ( present count ) after already playing one. Discards are always present count . This present count treatment is Bridge World Standard. In a NT contract , if you had K9752 & won the King , return your 4th best the deuce ( present count) . In a suit contract , you would return your 2nd lowest from 4 ( present count ). K972 , you win your king in a suit & return the deuce ( present count in a suit showing 3 ) . In NT , you would return the 7 ( MUD) when you play that treatment otherwise top of nothing with three. If you started with 3 to an honour your present count in both NT & a suit contract is the same a doubleton , so no problem.
My partner against 3NT lead 4th best from J10xx & AQ was on the board. I won the king from K9752 so I returned the deuce ( present count showing 4 ) . Partner now just applies a pattern 5-4-2-2 so knows to unblock with a spade honour so I can get my 5th spade to beat the contract. If I had an original K2 , I would switch as I know we are attacking declarers 5 card suit. In a suit contract , unblocking probably irrelevant.
Some more hints I got from Maksymetz in discussing players defensive cardings at the national level. Smith echoes are only useful in NT contracts. In suit contracts , if you cannot beat dummy’s card , Bryan suggests suit preference rather than count. It’s obviously not attitude , when you cannot beat dummys honour card ( Ace & King excluded ) so he feels suit preference is more useful than count in suit contracts only.
Bryan has strong opinions on “coded 9’s & 10’s”. He says they give too much information on the opening lead but use them during the middle of the hand helps the defense more than declarer. During the middle of the hand, it is more important to show whether you have a real interior sequence switch. Anyway these views are just his opinions.