A splinter is:
- a double jump shift by opener's partner, or
- a jump shift by opener following a single raise of opener's suit.
For example, 1♥ - 4♣ is a splinter by opener's partner showing shortness in clubs. 1♦ - 1♠ - 4♥ is a splinter by opener showing shortness in hearts.
A splinter always shows:
- Suit agreement with support, if it has not already been established;
- Game-forcing strength with slam interest;
- A void or singleton in the suit bid.
A splinter raise of a major shows at least 4-card support. Thus, in the sequence 1♥ - 4♣, at least a 9-card heart fit exists, and opener's partner has a void or singleton in clubs. In the sequence 1♦ - 1♠ - 4♥, opener shows exactly 4 spades and a void or singleton in hearts. (With 5 spades, they would have opened spades rather than a minor).
A splinter raise of a minor shows at least 5-card support.
Making a splinter bid
Splinters are usually bid with slightly less strength than what you would normally need for an invitation to slam; a minimum game-forcing hand can be enough for a splinter bid. Some partnerships have an agreement that a splinter bid is therefore limited in strength. With a very good hand, making a splinter bid can be disadvantageous since the information it conveys is less useful and yet it uses up a lot of bidding space. One could instead game-force with 2/1 or fourth suit forcing, then employ control bids and 4NT, Blackwood.
It's generally ill-advised to splinter if your short suit is a singleton ace or king; doing so would mislead your partner about the amount of strength you hold in the other suits.
Evaluate your hand carefully before responding. Partner is telling you that slam is viable if your hands "fit" with each other very well. The idea is that the splinter promises at most one loser in the short suit, so if your values are concentrated in suits other than partner's short suit, you have a good chance of being able to take twelve tricks in the other suits, losing at most one in the splinter suit. On the other hand, if you have values in partner's short suit instead of in the other suits, those high cards won't pull their weight, and the lower concentration of values in other suits may indicate losers in those suits.
Now proceed as follows:
I'm excited about the great fit we have and I think we can make slam → initiate control bidding or bid 4NT, Blackwood. Note that the splinter has already shown suit agreement, so there is no ambiguity about which suit is trumps.
I don't have slam interest and I have a stopper in the splinter suit → Sign off in 3NT.
I don't have a stopper in the splinter suit → Sign off in 4m.