[Tom E]

Why not bid 4♣ in response to my takeout double?

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[smerriman]

4♠ is the only possible call by North here. [edit - Stephen makes a good point - 4♥ might be even better.]

Unless North has strong hearts, North is forced to bid even with 0 points.

Since 4♣ could show 0 points and club length, South will have to pass a 4♣ bid.

If 4♣ makes, you'll score 130 points + 20 per overtrick.

If 4♠ makes, you'll score 420 points + 30 per overtrick.

4♠ therefore has substantially more to gain than it loses.

That's why to double as South you must have at least 3 spades (or a much much stronger hand that can bid again), and will actually have 4 most of the time.

That's also why North should choose 4♠ over 5♣ - minor suit games are much harder to make, and South is going to have 4 spades most of the time, making it the better choice.

[johnu]

I don't like the bid, but I would confidently balance with 3NT (Hamman's Rule). You can't double with a doubleton spade for the reasons that smerriman gave.

[Stephen Tu]

Agree 3nt by South way more sensible then double.
It could be argued that with only Jxxx in spades, and the cue bid being below 4S, that North should cue 4H to confirm whether South actually has 4, as if not I think I'd rather be in 5c. Maybe South has some more normal shape like 3154, 3244, and the spade 7 cd fit is just too weak to be playable?

In general though one is supposed to try really hard not to double with doubletons in unbid majors if other calls are at all reasonable.