[cnszsun]

Scoring: IMP

ps-1♣-ps-2♣
2♥-ps-3♥-dbl
ps-3♠-ps-4♠
AP

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[gnasher]

The final error was made by South.

If North had four spades (supposing that were allowed), he would have bid them over 3♥. Hence North can't have four spades, so South should convert 4♠ to 5♣.

[Codo]

If 2 ♣ denies 4 spades, this is obvious. And this is the standard approach.

[whereagles]

North's approach was of the "unlucky expert" kind.

With that flattish hand and South extremely likely to have a 12-14 balanced hand (he passed 2♥, remember?), North should reason the only game that can possibly make is 3NT, so he should just bid that. If they take the 1st 5 tricks... tough luck. But in that case you weren't going to make game anyway.