The negative double is used through {negative-double-max} promising four cards (at least) in an unbid major. Bidding a major in competition shows a five-card or longer suit and strength suitable to the level. With insufficient high-card strength for a free bid, double may also be used with a five-card or longer suit.
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1♣ — (1♦) — Double: 4–4 or better in the majors.
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1♦ — (1♥) — Double: exactly four spades (1♠ promises at least five).
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1♦ — (1♠) — Double: four hearts and 6+ points or five hearts and 5–6 points.
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1♠ — (4♦) — Double: four hearts and 10+ points or five hearts and 8-10 points.
If both majors are bid, then a double shows both minors.
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1♥ — (1♠) — Double: 4-4 or better in the minors.
When the opponents overcall a minor over a minor at the two-level or higher, a double shows at least one four-card major and at least two places to play.
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1♦ — (2♣) — Double: at least one four-card major.
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1♣ — (2♦) — Double: at least one four-card major.
If opener bids the a major you cannot support, rebid opener’s first suit or NT to imply the other major.