[2200]

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After W opened 1S， N led ♣Q against 6S, at both table.
At my table, ruffed at trick 1, declarer played ♠K and Ace, finding trumps 4-1, down 1.
At GIB's table, ruffed at trick 1, ♠K，♠9，covered by S!!!
17 imp swing.
I still don't understand, what makes bot cover?

[smerriman]

GIB plays as if the other players are playing double dummy. From a double dummy perspective, it makes no difference what card it plays; it's not clever enough to leave declarer a "guess".

[pigpenz]

what was the auction?
if north bid 3♣ there would be all the reason in the world to play it this way.
the normal C for opps would be 5579, so if north bids 3♣ it makes it more likely that
south would have 4 spades....look at Mckinnons book on bridge probablities.

[smerriman]

pigpenz, on 2020-April-04, 13:26, said:

if north bid 3♣ there would be all the reason in the world to play it this way.

You're missing the point - it's South's play that is a terrible play that no human would do, but standard for GIB; covering the 9 with the J guarantees declarer takes all the trumps.

[pigpenz]

smerriman, on 2020-April-04, 13:47, said:

You're missing the point - it's South's play that is a terrible play that no human would do, but standard for GIB; covering the 9 with the J guarantees declarer takes all the trumps.

yes I understand that but still would like to know if there had been any bidding???

[johnu]

pigpenz, on 2020-April-07, 10:51, said:

yes I understand that but still would like to know if there had been any bidding???

If there was no bidding, there wouldn't be a contract to play

In the actual case, West has opened the bidding 1♠ which promises 5 spades. From GIB's perspective as South, partner must have a singleton spade so there is no possible spade trick because declarer will finesse (double dummy play) the jack. Therefore South mistakenly thinks it doesn't make any difference whether South covers the spade or not.

This is one of the huge weaknesses of GIB because declarer would normally play for a 3-2 spade break and end up losing a trick to the jack, but South has randomly thrown away a trick by a useless and hopeless cover on the second spade trick.