Double 1NT (12-14) for penalties with 15+pts and reasonably balanced hand. If they try to escape the double, then a further double is for penalties. If you as the doubler’s partner hold less than 5 pts, and a 5 card suit, bid that suit, rather than leaving the double in.
Doubles of suit opening bids: are for take-out up to 3 spades. Includes weak twos, and pre-emptive three bids. A second double of the same suit is usually for penalties. Should show a shortage in opponents suit, or very strong hand. Example:opponents open 1 you hold 17 pts and a 6 card heart suit, double first, then bid hearts later
If opponents have a bid suit which is supported, doubles are for take-out to 3 spades
If partner opens, and is overcalled, a double by you is for take-out, shows 9+pts, and suggests the other two suits, but is not limited to having these. You may hold the opponents suit intending to bid no-trumps later.
If the opponents bid two suits then a double shows the other two suits
If opponents open with a suit and the responder bids 1NT, a double by you is for penalties with a strong running suit in your hand and entries
If opponents double partners opening suit, and you can support this suit, bid one more in the suit than you would have done, had there not been a double. Example: 1 dbl 3 shows a hand where you would have bid only 2
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