A negative double promises support for the unbid suits and shows a hand where you don’t have a natural bid to make. It means that you can take part in the auction safely even with quite a weak hand.
You need 10 HCP or more to change suit at the 2-level and you can’t support partner. You can’t bid 1NT without a spade stopper. The solution is to make a negative double. The message to partner is that you have some points, some length in the unbid suits, but not enough strength to bid at the 2-level.
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