RESPONSES TO OPENING 1S

PassVery weak (0-5 points), no desire to shut out opponents.
Double of suit overcallTake-out if below 4S. To make a takeout double of an overcall and then pull partner's response when they do take it out is non-forcing with at least a five-card suit, since an immediate bid would have been forcing. (The logic is different after 1NT openings.)
Double of no-trump overcallPenalties
Redouble10+, wants to double for penalties, often a misfit. During the subsequent auction, the partnership has one standard takeout double "in the bank". This is in order to distinguish between a hand that simply wants to compete (will pass and wait for partner to make a takeout double) and one with game-forcing strength (will bid directly, forcing).
1NTUnable to make any other response, says nothing about shape
Minimum cue bid of overcall where the partnership can still stop in part-score; i.e. below 3 of opener's suitUnassuming Cue-Bid, with trump support and intermediate points. (Might occasionally be a preliminary move on a much stronger hand.)
New suit at 2 level after 1NT overcallNatural, non-forcing and to play, because a hand with game-going strength would have doubled. Auctions like 1S (1NT) 2C from a passed hand no longer show a weak hand with three-card trump support because that's too dangerous after the 1NT overcall, while intermediate hands will just double.
2 of a minorGame-forcing, assumed to be 4 cards but occasionally a waiting bid.
2C if passed hand3-card support for partner, 2-level values and no better bid available. This still applies after a suit overcall or a double, but after a no-trump overcall it's natural and non-forcing, with clubs.
2D if passed hand3-card support for partner, intermediate points and no better bid available. This still applies after a suit overcall or a double, but after a no-trump overcall it's natural and non-forcing, with diamonds.
2H if passed hand3-card support and a five-card heart suit, intermediate points. This still applies after a suit overcall or a double, but after a no-trump overcall it's natural and non-forcing, with hearts.
2HGame-forcing, 5 hearts
Raise to 2Weak, with trump support. If only three, had no better bid and in particular didn't much like the look of any of the less marginal responses (i.e. 1NT).
Raise to 2 after overcallWeak, with trump support. If only three, had no better bid.
Raise to 2 after doubleWeak, promises no strength at all
2NT11-12, balanced, would have raised a weak 1NT to two. Denies four-card spade support. Also promises precisely a doubleton in partner's suit if a passed-hand 2D response had been available to show intermediate strength with three-card trump support. Any continuation in an uncontested auction is game-forcing.
2NT after overcallNatural, constructive, about 11-12 points. Good holding in the opponents' suit, preferably at least one and a half stops.
2NT after doubleIntermediate jump raise in partner's suit. (Might occasionally be a preliminary move on a much stronger hand.)
Jump shiftStrong six-card suit, 16+ points. Still applies after any intervention except a 1NT overcall.
Jump shift by passed hand or after 1NT overcallFit-Jump, two-suited, at least 5-4, trump support for partner. Forcing and normally shows an 8-loser hand or so, without enough strength to go to game.
New suit response from a passed hand above 2 of opener's suit (after opponents' intervention)Whatever the same bid would have meant without the intervention, e.g. pass (pass) 1C (1S) 2H is a fit-jump in hearts, just as it would have been without the overcall.
Jump raise4-card support, intermediate (10 points or so)
Jump raise to 3 after interventionPre-emptive. A redouble or an Unassuming Cue Bid is the bid to show an intermediate raise.
Unnecessary jump shift up to Splinter LevelSplinter, agreeing partner's trump suit and showing a small singleton or void. Splinters are normally game-forcing, but if two of them were available then the lower is merely invitational, e.g. 1C 1S 3D/4D are both splinters, since 1C 1S 2D would have been a reverse.
3NTstrong no-trump (i.e. balanced 15-17) with four-card trump support.
Game raise (opponents silent)Shows shape rather than points, might be pre-emptive.
Game raise (contested auction)To play.
Jump cue bid after overcallSplinter
Jump shift to Blackwood LevelExclusion Blackwood
Jump shift to Grand-Hunting LevelComplete two-suiter. If the partnership had previously bid more than one suit and so there's ambiguity about which suits are being promised, then the other one is the first-bid suit of the player who just jumped.

DEFENCE TO 1S

Any bid in the pass-out seatA king weaker than when bid directly.
DoubleOne of two hand types. DEFAULT ASSUMPTION: take-out, opening strength, support for unbid suits. This is a "say it once and shut up" bid that thereafter leaves all competitive decisions to partner and the doubler is not expected to bid again unless partner finds a bid that asks them to do so. To do so shows extreme strength (17+) and previous assumptions about shape no longer apply. NOTE: this bid promises proper opening values (i.e. you'd been going to open the bidding if the opponents had passed) AND support for the unbid suits. Opening a 4441 10-count "because the shape is perfect" is a violation of all that is good and will cause cancer in kittens.
Cue bid of majorMichaels, 5-5 shape, the other major and a minor
Suit overcall5-card suit, strength depends on vulnerability and level
Jump suit overcallWeak Two, 6-10 and a 6-card suit
Suit jump in the pass-out seat10-14 actual points (i.e. no transferred king), 6+ trumps, denies a four-card major side-suit, like a fourth-seat 2H opening.
Suit jump in spades in the pass-out seat8-12 (i.e. no transferred king), 6+ trumps, denies four hearts, like a fourth-seat 2S opening.
Greater jump suit overcallPre-emptive
1NT from a passed hand (protective)Stretching with a good eleven-count and the usual transferred-king principle.
1NT from a passed hand (non-protective)5-5 shape and a distribution that can't be shown with either Unusual 2NT or Michaels, e.g. the black suits after a diamond opening. If there's no such unshowable distribution (i.e. after a major opening), it's the LOWER of the two Michaels options, i.e. (1S) passed-hand-1NT would show clubs+hearts.
1NT15-17, natural, with a useful holding in any opposition suits. Responses are the same as after a 1NT opening bid.
2NT5-5 in the lowest two unbid suits, Unusual 2NT
Jump to 3-level cue bidasking partner to bid 3NT if they have a stopper in this suit
Jump to 3NTTo play, long running minor.
4NTAcol 4NT, asking for specific aces
5NTasking for specific kings
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Copyright 2015 Finn Clark.