Conventions in depth
A double jump in a new suit that agrees partner’s major, forces to game and shows a singleton or void in the suit bid.
A splinter is a game-forcing raise with about 13–16 support points and slam interest, and shows a singleton or void in the suit bid:
Because it lets partner judge whether the high cards held are working or wasted. Honours opposite the shortness (a king or queen in the short suit) are largely wasted; honours in the other suits count in full. Opposite the right hand, opener may drive on, often via Roman Keycard Blackwood.
Partner weighs the high cards in hand against the announced shortage and either drives on or settles for game:
The double jump keeps a splinter from being confused with a natural suit or a control cue-bid. With no slam interest at all, you would bid game instead.
The same jump is not confined to the plain 1M-then-jump sequence. It can be used whenever a major fit has just been agreed, in auctions that opened with a minor and in competition as well as after a 1♥ or 1♠ opening. The shortage and the game force mean the same wherever it appears.
Over partner’s 1NT the jump is available once a transfer has been accepted, with the roles the other way round: the trump suit is responder’s major, so the bid promises a six-card or longer suit rather than four-card support.
Opener judges it exactly as above, counting honours opposite the shortage as wasted, and can ask for keycards; the trump suit is responder’s major, so the answer counts that suit’s king and queen.
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