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A bid, on the second round, of a double jump shift or more, shows 4 card support for partner's suit and two or three cards in the suit bid. It implies shortness
in one of the unbid suits.
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South North
1C 1H
3S
3S shows four card heart support plus two or three spades (usually three), and implies a singleton or void in diamonds.
South North
1S 4C!
4C shows four card spades support and two or three clubs, as well as implied shortness in diamonds or hearts.