Negative doubles are an essential part of a 5 card major system. Their purpose is to allow a partner ship to find a 4-4 major fit after an opponent has bid. Negative doubles are only used in competitive auctions. A negative double is a takeout double for the two unbid suits, showing four cards in each (usually). Only make negative doubles when partner has bid and the opponents overcall. You must have 4 cards in any unbid major. You must have at least 8 hcp to make a negative double.
Some illustrative examples:
If you are playing negative doubles, you double. This tells your partner that you have 4 spades and probably 4 clubs. You would make the same bid with Axxx and Qxx, because if partner bids clubs, you can go back to diamonds.
Here, you do not have four cards in the other major, so you cannot make a negative double. Just support partner's diamonds by bidding 2.
Remember: when one major suit has been bid, a negative double always shows the other major suit.
There is a problem here. You have 5 spades. A negative double shows only 4 spades. With a 5 card suit, bid your major. With this type of hand, bid 1. Remember, with a slightly better than minimum hand and 5 spades, it is fine to bid 1.
You are just as strong here as in the above hand, but bidding 1 would show 5 spades and you only have 4. Negative double and then bid again after partner bids.
Because you have both major suits and at least 8 points, negative double. You show your majors to partner who can then decide where to play the contract. With one fewer heart or one fewer spade, you need 10 points to negative double because you don't have 4 cards in each major.
Because you don't have both major suits, you cannot make a negative double. So just pass. If partner has anything to say, he'll let you know.
Even though you have 17 pts and are interested in slam, you should still negative double. Your partner can't pass it and if you bid 1 spade, it will show a 5 card suit. After your negative double, make forcing jump bids or bid the opponents suit until you are in game or slam. Partner will eventually find out about the strength of your hand, although not right away.
Negative double. This will tell partner that you have diamonds and hearts. Just because partner can't rebid the suit on the one level doesn't mean negative doubles don't apply.
You can't negative double because that would show spades and you have hearts. Bid 1NT, showing better than nothing and a stopper in the opponents suit.
Until now, you would have been shut out by your opponents preempt. But no more. Negative double. It tells partner that you have 4 hearts and would have had enough points to bid on the 2 level, had you been given a chance.
Negative double again. You don't want to miss a likely game, and you still don't know the shape of partner's hand. Tell him what your hand looks like by negative doubling.
Here you pass. You don't have enough points to force partner to the 4 level, so you give up on the 4-4 heart fit and try to set 3. With a really great hand, partner will bid again and then you can bid 4.