If you adopt this strategy, then your use of the 1♦ opening has to be adjusted accordingly, so that you open 1♦ with hands unsuitable at that vulnerability for a 1NT Opening. In practice, you can use whatever strength you like for your Opening 1NT bid. Some Precision pairs use a variable No Trump Opening, 10-12 when not vulnerable and 13-15 when vulnerable. This would affect the 1♦ Openings but no other part of the system. Any increase in the top of the range for an Opening 1NT will affect the 1♣ Openings, of course, and would be discouraged.
Even when non-vulnerable, in 4th position (after 3 passes), 1NT is never showing 10-12, but always 13-15 in principle (No point in a semi-pre-emptive opening when everyone else has already passed).
The OCP scheme of responses to 1NT uses 4-way Transfers & non-promissory Stayman, a 2-way 2♦ bid, Transfer Lebensohl over interference, and INTRO when 1NT is doubled for penalties.
The basic scheme of responses to 1NT follows. This scheme of responses also applies when Opener has opened 1♣ and rebid 1NT (via the Cambridge Heart Complex or not), but, in the Simple System, does not apply to a 1NT rebid after a 1♦ Opening. A different scheme of responses apply over Opener's strong balanced 2NT and 3NT Rebids (See the 1♣ page):
Non-promissory Stayman. Says nothing about Responder's hand at this stage and simply asks partner to bid a 4-card Major if they have one, and 2♦ otherwise. No other responses are permitted. See the section on Stayman
This is either...
A normal Transfer to Hearts, or
The prelude to an invitational sequence.
See the Section on the 2-Way 2♦ below.
A Transfer to Spades
A Transfer to Clubs
A Transfer to Diamonds (Note:NOT a balanced invitation). See the section on 4-way Transfers below.
A transfer to Hearts with no interest beyond game. The sequence 1NT-2♦-2♥-4♥ is a mild slam try and Opener should start cue-bidding with a good fit and controls.
A transfer to Spades with no interest beyond game. The sequence 1NT-2♥-2♠-4♠ is thus a mild slam try and Opener should start cue-bidding with a good fit and controls.
A very distributional Minor 2-suiter (at least 6-5), but essentially weak and pre-emptive with no slam interest.
(Quantitative but never used. See the 2-way 2♦ below)
This is one of the cornerstones of any 1NT Opening and is usually about the first convention that beginners are taught. OCP uses 4-way Transfers (ie: 1NT-2NT is a transfer to Diamonds), so invitations in NTs have to go either via Stayman or via the 2-way 2-way 2♦. The upshot of this is that in OCP Stayman is "non-promissory" meaning that Responder does not have to hold a 4-card Major in order to use Stayman. This has an effect on some of the sequences.
A lot of rubbish has been written about Stayman in the past, saying you need to have such a point-count in order to use it, etc. In practice the only requirement is that Responder can cope sensibly with any of the 3 allowed Opener's rebids. If any one of Opener's possible responses to the Stayman enquiry would embarrass Responder, then Stayman should not be used.
A bid of 2♣ simply asks Opener if their hand contains a 4-card Major suit. In response
denies possession of a 4-card Major suit. Now:
shows that Responder has bid so-called "Garbage Stayman" with a weak hand with something like 4450 shape (void in Clubs and 5-card Diamonds), a classic use of Stayman where Responder passes whatever Opener responds to the 2♣ Enquiry.
is weak and "to play", with 5-card or longer Hearts and 4-card Spades
is weak and "to play", with 5-card or longer Spades and 4-card Hearts
in OCP this is mildly invitational to 3NT, wanting to be in game only opposite a maximum 1NT Opening.
Note: Contrast the sequence using the 2-Way 2♦ shown below where Responder wants to be in game if Opener is better than minimum.
in OCP this shows the values for game but concern about one of the other suits, usually Diamonds, but occasionally about a Major. Stops are bid upwards from this point. If, however, Responder is a passed hand or it is clear that game values may not be present (eg: in a 1♣ sequence with a negative response), then this sequence simply shows a 5-card or longer (usually at least 6-card) Club suit with a 4-card major, and is just "to play".
in OCP this shows the values for game but concern specifically about Clubs. Opener bids 3NT with decent Club values or may bid a 3-card Major otherwise. If, however, Responder is a passed hand or it is clear that game values may not be present (eg: in a 1♣ sequence with a negative response), then this sequence simply shows a 5-card or longer (usually at least 6-card) Diamond suit with a 4-card major, and is just "to play".
is invitational with 5-card or longer Hearts and 4-card Spades. Continuations natural.
is invitational with 5-card or longer Spades and 4-card Hearts. Continuations natural. Note: You may choose to use Smolen here
is natural and "to play".
shows any hand containing a 4-card Heart suit, even if it also contains a 4-card Spade suit. Continuations are as follows:
Weak with both Majors, normally at least 5-4
shows a 4-card Spade suit but is forcing. It denies having 4-card Hearts, but does promise the values for at least a raise to 2NT. Opener bids 2NT or 3♠ with any minimum or mid-range hand (depending on whether or not they have a 4-card Spade fit) or 3NT or 4♠ with a maximum hand. Note: This is the major difference when playing non-promissory Stayman. Responder must show the Spades if they have them. A 2NT rebid by Responder over 2♥ would be just a mild invitation to 3NT with no 4-card Major.
In OCP this is mildly invitational to 3NT with no 4-card Major, wanting to be in game only opposite a maximum Opening 1NT. Contrast the sequence using the 2-Way 2♦ where Responder invites Opener to show whether or not they are better than minimum.
in OCP this is forcing and denies a 4-card Major, showing the values for game but concern about Spades or Diamonds (and usually implies a 5+-card Club suit). Stops are bid upwards from this point. Note: If Responder is a passed hand or it is clear that game values may not be present (eg: in a 1♣ sequence with a negative response), then this sequence simply shows a 5-card or longer (usually at least 6-card) Club suit with a 4-card Spade suit, and is just "to play" (or Opener can correct to 3♠.
in OCP this is forcing and denies 4-card Hearts, showing the values for game but concern about Clubs specifically. Opener bids 3NT with decent Club values or may bid a 4-card Spade suit otherwise. Note: If Responder is a passed hand or it is clear that game values may not be present (eg: in a 1♣ sequence with a negative response), then this sequence simply shows a 5-card or longer (usually at least 6-card) Diamond suit with a 4-card Spade suit, and is just "to play" (or Opener can correct to 3♠)
A natural invitation to game in Hearts, but still only wanting to be in game opposite a maximum 1NT Opening.
is an Eta Ask in Hearts. Asking Bids now established.
shows exactly 4-card Spades and values for game. Note: This sequence does show 4-card Spades, because Responder would have no reason to use Stayman otherwise.
shows 4-card Spades and denies 4-card Hearts. Continuations are as follows:
Weak with both Majors, normally at least 5-4
This denies 4-card Spades (may have 4-card Hearts) and in OCP is mildly invitational to 3NT, wanting to be in game only opposite a maximum Opening 1NT.
in OCP this denies 4-card Spades, shows 4-card Hearts and a 5-card or longer Club suit in a hand of invitational strength. Continuations are natural.
in this system this denies 4-card Spades, shows 4-card Hearts and a 5-card or longer Diamond suit in a hand of invitational strength. Continuations are natural.
is an Eta Ask in Spades. Asking Bids now established
A natural raise in Spades, but only wanting to be in game opposite a maximum 1NT Opening.
shows 4-card Hearts and values for game.
Note: It can be seen from the above that there are specific circumstances where Stayman is more appropriate than the 2-Way 2♦ sequences, and that Stayman is also more appropriate with invitational hands than transfers, since the showing of a second suit after a
transfer is always forcing. Similarly on other hands, the 2-way 2♦ is more appropriate.
Classic Stayman does not use Transfers, but instead relies on double-barrelled Stayman, which I always found cumbersome, unintuitive and awkward. Instead, OCP uses 4-way Transfers (ie: 1NT-2NT is a transfer to Diamonds) because, in our view, transfers are such an incredibly useful device that it is criminal not to take advantage of them for all four suits equally.
Using OCP, we always transfer into a 5-card or longer Major, even if semi-balanced and with a relatively poor suit. With 5-card Minors, we normally only bother with transfers if we also have a second suit (and we're strong enough to show it). With a single-suited 6-card or longer Minor we normally only use a transfer if weak, very unbalanced with no intention of playing in 3NT, or of slam-invitational strength. With a game-going single-suited Minor that's happy to play in 3NT, we tend to just bid 3NT. In OCP Transferring to a Minor and then bidding 3NT is a mild slam try and Opener will proceed beyond 3NT if maximum, with a fit for the Minor and a ruffing value elsewhere (ie: not 4333)
Note:In OCP the sequence 1NT-2♦-2♥-2♠ does not show a Major 2-suiter with longer Hearts, but is part of the 2-Way 2♦ sequences. To show the major 2-suiter, responder must rebid 3♠.
No Trump Rebids after Transfers
A 2NT or 3NT rebid after a transfer to a major simply shows a semi-balanced hand with a 5-card major and
gives Opener the option as to where to play. A 3NT rebid after a transfer to a Minor suit (except where
Opener has broken the transfer - see below) conveys a different message because a semi-balanced hand with
a minor suit tends to be treated as balanced. Now this is a mild slam try showing an upper-range raise to 3NT.
Opener can continue past 3NT if maximum themselves with good controls, a good trump fit and a hand that is
A Super-accept is any action where Opener does not complete the transfer as requested. Super-accepts always show strength and an excellent fit for Responder's suit. They are never bid because of a poor fit for partner's suit. Breaks of transfers to a Major always promise decent 4-card support for Responder's suit, a maximum hand, and good controls. Breaks of transfers to a Minor do not promise 4-card support, since the emphasis is normally on reaching a 3NT contract in these circumstances unless responder is very strong, but they do promise at least 3-card support, normally with at least 1 of the top three honours, a maximum hand outside and decent "pure" values outside. A break of a transfer is always initially saying that Opener would accept even a mild invitation to game.
Breaks of Transfers to Majors
Over transfers into a Major suit, Opener has two commonly accepted options for showing a Super-Accept:
They bid the cheapest side-suit first-round control they hold
They show a "worthless doubleton" (ie: a suit with xx) if they have one.
OCP uses both of these methods. We recommend that over a transfer to Spades you super-accept by cue-bidding your cheapest side-suit Ace, but over a transfer to Hearts (ie: the 2-way 2♦), you should super-accept by showing a "worthless" doubleton (xx) if you have one. The reason for this difference is because After a transfer to Spades, we know Responder will always have a 5-card or longer Spade suit. Responder using the 2-way 2♦, however, may not have a 5-card or even a 4-card Heart suit and Opener showing where they are particularly weak (Jx at best) will, while the information might assist the defence, at least give us the best chance of arriving in a sensible contract and avoid 3NT contracts where we have matching weaknesses.
Showing Worthless Doubletons
OCP's general strategy is to bid the suit below any shortage (eg: Splinters, Mini-splinters etc), and to keep this policy we recommend you do the same when showing a worthless doubleton. Not only does his permit a cheap Beta Ask in the short suit if Responder is very strong and has Hearts, but also it keeps our approach consistent across the System as a whole. There is only a small, occasional advantage to be gained from this, however, and you do not have to follow this strategy if you prefer not to
If, over 1NT-2♦, Opener has a hand worthy of a super-accept of Hearts, but doesn't have a "worthless Doubleton", they bid 2NT with any 4432 Shape or 3♥ with 3433 shape, or can even decline to super-accept.
If Responder does not have even a 3-card Heart holding then they take any appropriate action that does not involve supporting Hearts. This will normally involve bidding No Trumps at an appropriate level, but over anything other than 1NT-2♦-3♥ they can choose to start bidding 4-card suits upwards since Opener is guaranteed to hold a second 4-card suit in addition to Hearts. In this situation Opener needs to bear in mind that Responder may or may not have a 4-card or longer Heart suit and should avoid a jump to 4♥ prematurely. (See the 2-Way 2♦ below).
(3) Breaks of Transfers to Minor Suits
Breaks of transfers to Minor suits are always achieved in the same way which is by bidding the suit immediately below the transfer suit (ie: the intervening suit), ie:
1NT-2♠-2NT rather than bidding 3♣, and
1NT-2NT-3♣ rather than bidding 3♦
In this way a Responder with a very weak single-suiter can cheaply refuse the invitation by converting to their Minor at the 3-level.
As you will see from the 1NT Summary at the top of the page the sequence 1NT-2♦ has two possible meanings, being either
A normal transfer to Hearts, or
The prelude to an invitational and/or exploratory sequence
This is a convention I devised and licensed back in the UK in the early 1980's and is achieved in the following way:
Opener's Initial Action
Opener initially assumes that the 2♦ bid is a transfer to Hearts and bids accordingly (including super-accepting as above if necessary, since this gives Responder most of the information they are after anyway). If Opener does not break the transfer, then any rebid by
Responder other than 2♠ confirms the Heart transfer and bidding proceeds exactly as normal.
The special sequence only kicks in if Opener doesn't super-accept and Responder then rebids 2♠. The 2♠ rebid is made with three possible hand-types:
A normal invitational hand where Responder wants to be in 3NT opposite a better-than-minimum hand, or
A game-going hand with a significant weakness somewhere (eg: 4441, but maybe 4432 or even 1345) but no long suits, where Responder wants to check for potential matching weaknesses before signing off in 3NT. A hand where a suit game (even in a Minor) might be preferrable
A much stronger responding hand interested in a possible slam and wanting to find out more about Opener's range and exact distribution cheaply before bypassing the 3NT level. This hand-type is almost never single-suited except 5332 with a Minor.
The 2♠ rebid initially asks Opener to do one of two things:
To Rebid 2NT with any minimum hand, or
To do something else with any hand that is better than minimum.
Opener's Action over 2♠
If Opener rebids 2NT
If Opener rebids 2NT and Responder has hand-type (1) above (ie: the strongly invitational hand), then Responder simply passes, since Opener has shown a minimum hand.
Any other action by Responder shows hand-types (2) or (3) above and is game-forcing. The fact that Opener is minimum does not necessarily mean that Responder has given up on the possibility of a slam. If Responder continues beyond 2NT in this fashion, then they normally start bidding 4-card (or longer) suits upwards at the 3-level in search of a fit. Now suits are bid upwards towards 3NT and 3NT is bypassed by either partner when/if a fit is found in a Major, or by Responder if a fit is disclosed in a Minor (the general rule is that Opener doesn't go past 3NT unless we find a 4-4 Major suit fit). This can be done by explicitly agreeing the suit at the 4-level or by cue-bidding the cheapest 1st-round control held at the 4-level (whichever is the cheapest - See the examples below). If Responder simply rebids 3NT over 2NT then this tends to show a very strong but flat hand with a 4-card Minor no longer interested in slam.
If Opener does "something else"
The "something else" normally consists of Opener bidding his lowest 4-card suit at the 3-level. Continuations are now pretty much identical to those where Opener signs off in 2NT and Responder continues (see above)
The other option is for Opener to rebid 3NT rather than 2NT and this shows a hand with 3334 or 3343 shape (ie: with a 4-card Minor).
With 5332 shape, Opener should always show it. Thus a sequence like 1NT-2♦-2♥-2♠-3♦-3♥-3NT will always be a 5-card Diamond suit, because 3♦ denied having 4-card Clubs and clearly Opener doesn't have a 4-card Major, but nevertheless elected to show the Diamonds.
Note 1: If Opener shows a better-than-minimum hand by bidding a suit at the 3-level then 4-card suits continue to be bid upwards towards 3NT. The difference here is that Opener is not allowed to bypass 3NT unless a Major-suit fit is found, since Responder may only have the normal invitational hand. Responder is allowed to go beyond 3NT to show a Minor suit fit. As above a fit is shown by either explicitly agreeing it at the 4-level or by cue-bidding the cheapest 1st-round control held.
Note 2: A 3NT rebid by Responder is always "to play". If Opener ends up bidding 3NT and Responder now bids 4NT, this asks Opener to show any 4-card Minor-suit fit that has been "concealed" (because Opener is not allowed to bypass 3NT) and Opener shows the suit or passes (ie: Responder cannot go this route if they want to play in 6NT regardless).
If Opener rebids 3NT rather than 2NT
If Opener rebids 3NT to show a better-than-minimum hand but with 3334 or 3343 shape, then with the normal invitational hand, Responder simply passes. Any other action shows the very strong hand. Now:
4♦ asks Opener to bid 4♥ with 4-card Clubs, and 4♠ with 4-card Diamonds.
Any other bids are natural and "to play" or invitational.
Examples of 2-way 2♦
Some of these sequences are complete, but some are open-ended and would continue beyond where the sequence shown stops.
Confirms the Heart Transfer, a semi-balanced invitation.
Confirms the Heart Transfer, a Heart/Club 2-suiter, forcing.
Confirms the Heart Transfer, a Major 2-suiter with longer Hearts, forcing
A mild slam invitation in Hearts (a hand not remotely interested in slam would go the 1NT-4♦ route)
Opener here must be better-than-minimum, 5332-shape with Diamonds. They have no 4-card Major and have also denied having a 4-card Club suit by bypassing it. With 3343 shape they would have rebid 3NT rather than 3♦.
Opener here clearly has 4-card Diamonds and 4-card Hearts. The 4♣ agrees Hearts and shows a Club control. Responder can convert to 4♥ with any normal hand or continue the cue-bidding sequence with a slam-invitational hand.
Opener here is minimum (2NT rebid) and Responder has a game going or slam-invitational hand. Responder has shown 4-card Hearts and denied a 4-card Minor. Opener has denied a 4-card Major. From here Responder can accept defeat and pass, show a second suit with a big 2-suiter or bid on towards 6NT.
Opener here is minimum (2NT rebid) and Responder a strong hand-type. Responder has shown 4-card Diamonds and Spades and denied a 4-card Heart suit. Opener has now agreed Diamonds and is inviting Opener to cue-bid. 5♦ instead of 4♦ would obviously just be "to play". 4♣ instead of 4♦, however, would be a cue-bid agreeing Spades.
Here Opener has shown a better-than-minimum flat hand with 3NT. Responder has shown a strong hand-type by continuing with 4♦, which asks Opener to clarify their shape and 4♠ from Opener shows 4-card Diamonds, (the 3NT rebid denies a 4-card Major).
For complete details on Transfer Lebensohl (and Lebensohl) please see the Lebensohl page specifically covering this topic. What follows is a brief description of Transfer Lebensohl, which is what we recommend that you play when opponents interfere at the 2-level over your 1NT opening.
Interference over 1NT can make action by Responder difficult if you are not using Lebensohl of some kind: New suit bids at the 3-level over interference have to be either "to play" or invitational or forcing but cannot be all three at the same time. Similarly it is difficult for Responder to explore for a 4-card
Major-suit fit safely and at the same time show or deny a stop in the opponents' suit (in case a fit is not found). Transfer Lebensohl overcomes all of these potential problems. Transfer Lebensohl uses the 2NT bid as a special kind of transfer, simply instructing Opener to bid 3♣. This now creates 2 kinds of sequences, ones that do go via this 2NT-3♣ relay sequence (called "Slow" sequences), and ones that do not (called "Fast" sequences).
When opponents overcall a 1NT Opening at the 2-level, then action by Responder is as follows:
2-level suit bids are merely competitive and "to play" with no interest in game.
2NT is the "Lebensohl" bid and commences what are called "slow" sequences. At this stage 2NT says nothing whatever about their hand. It simply commands Opener to rebid 3♣, after which Responder will clarify their hand-type.
Now after 1NT-(2x)-2NT-3♣-??
Pass shows a hand merely wanting to compete in Clubs
A "Slow" new suit bid at the 3-level is invitational whether or not it could have been bid at the 2-level. It says nothing at all about Responder's holding in opponents' suit
A "Slow" cue-bid of the opponents' suit shows values for 3NT, 4-card interest in any unbid Majors and also promises a stop in opponents' suit.
A "Slow" bid of 3NT shows values for game, denies any interest in unbid Majors, but promises a stop in opponents' suit.
Immediate bids at the 3-level are called "fast" bids and, with the exception of 3NT, are all transfers into the next suit up. In all cases it is the suit that is being transferred into that matters, not the suit that is being bid:
Transfers into a new suit are either weak or game-forcing (if the suit could have been bid at the 2-level, then they are treated as being at least strongly invitational or forcing). Opener completes the transfer and Responder then passes with a weak hand or bids again with a forcing game-going variety. Normal arrangements for breaking transfers apply. When Opener completes the transfer, Responder is able to take whatever action they like, to pass, bid 3NT, show a second suit or bid game in their first suit. Note: if the suit Responder is showing is higher-ranking than Opps' suit, and the transfer is always at least strong invitational, then Opener should only merely complete the Transfer at the 3-level if they would not accept that strong invitation. If they would accept it, they should bid game or cue-bid below game level.
A transfer into Opponents' suit is a "fast" cue-bid of their suit, showing values for 3NT and interest in any unbid Majors, but denying a stop in Opponents' suit (Whereas the "slow" cue-bid promises a stop). Bidding from here is natural. Opener can show any unbid Major they hold or bid 3NT naturally if they have values in opponents' suit. If they have no Major and no stop, then they can bid a 3-card Major or a 4-card Minor. Note: a fast 3-level bid of Opps' suit is a transfer into the suit above, not a cue-bid of Opps suit. Thus:
1NT-(2♥)-3♥ is a transfer to Spades, and
1NT-(2♥)-3♦ is the cue-bid of Hearts
A "fast" 3NT shows values for game, denies interest in unbid Majors and denies a stop in Opponents' suit. Typically, it shows a long running Minor with values in the Majors. Opener passes with a stop in opponents' suit or converts to 4♣ otherwise.
A "fast" transfer into Diamonds followed by a cue-bid of opponents' Major suit specifically shows a running Diamond suit and exactly a half-stop in their Major, asking for additional assistance in the suit.
Unfortunately this option is not available with a Club suit below the level of 3NT, but is available at the 4-level:
Coping with Artificial Interference
Once again, this is a relatively brief summary only. For fuller details please see the page specifically on Lebensohl
In practice, artificial interference against 1NT Openings is almost the norm. Here are some fundamental guidelines:
Showing Single-suited hands
If Opps interference shows a single-suited hand but doesn't specify the suit (eg: DONT or Brozel Double, Capp 2♣), then normally it's safe for Responder to Pass because they will get another opportunity to bid. Their LHO will normally relay in 2♣ (2♦ if Capp) and RHO will pass or correct.
Showing 2-suiters, only 1 specified
If Opps' interference shows a 2-suited hand but only specifies one of the two suits, then essentially we ignore the unspecified suit entirely. Cue-bids relate to the suit Opps have actually shown, regardless of what they actually bid.
eg: 1NT-(2♦(Spades & another)-??? Now cue-bids by Responder are cue-bids of Spades, rather than Diamonds. Often "fast" cue-bids are available at the 2-level, which should be used, to save space.
Showing 2-suiters, both specified
If Opps' interference shows a 2-suited hand and specifies both of the suits, then cue-bids relate to the suits Opps have actually shown, regardless of what they actually bid.
eg: 1NT-(2♦(Majors)-??? Now cue-bids by Responder are cue-bids of either Major, rather than Diamonds. Often "fast" cue-bids are available at the 2-level, which should be used, to save space. Note: We always promise a stop in any of their suits that we bypass in order to cue-bid the other.
eg: 1NT-(2♦(♦ + ♥)-3♣ Here 3♣ is a "fast" cue-bid of Diamonds (transferring into their Diamonds), so we are denying a stop in Diamonds, but in the process we've also bypassed a 2♥ cue-bid of their Hearts, so we're promising a Heart stop, and probably have 4-card Spades into the bargain (because cue-bids tend to be Staymanic.
If the 'normal' version of Suction is in use (ie: 1NT-(2♣) shows either single-suited Diamonds or both Majors), then essentially we're in a similar position as over Brozel & DONT-style doubles, because the overcaller will never have the suit they actually bid, and their partner will almost always relay in the potential single-suited suit, and the overcaller will then pass or correct with the 2-suited hand. It's therefore normally safe to Pass initially until you know exactly what Opps have, and then decide whether to compete (or whatever).
As you can see, Transfer Lebensohl allows almost all possible hand-types to be shown. The only exceptions are invitational-strength hands with Clubs and hands with long Clubs and a half-stop in Opponents' suit. You can always show interest in any unbid Major suits and at the same time show or deny a stop in the opponents' suit. Competitive and strong hands can always be shown and the sequences are flexible enough to be able to cope with most contingencies.
INTRO (1NT Redouble=0)
This is an escape mechanism when opponents double your partner's 1NT Opening. If, as OCP does, you are sometimes opening 1NT with 10-12 hcp, it's absolutely essential to have an escape mechanism such as INTRO (but there are loads of other ones, so take your pick). A lot of people just play "System Off" once opps double their 1NT Opening, otherwise Responder has no easy escape into a long Minor suit. INTRO goes a fair way further than that. The scheme of actions by Responder using INTRO is as follows:-
Immediate 2-level suit bids are weak and "to play", promising a 5-card or longer suit
Immediate 3-level suit bids are invitational, promising at least a 6-card suit in an unbalanced hand where Responder has at least 1 shortage and no desire to play in 1NTX.
Immediate bids from 3NT up to 4♥ are all transfers with game-going single-suiters.
Redouble shows a weak hand with either 4333 or 4441 distribution and asks Opener to bid their cheapest 4-card suit. Responder then passes with any 4333 hand or bids the next suit up if they have a 4441 type and Opener has bid their singleton. Bidding stops as soon as at least a 4-3 fit is found.
Optionally you can add an invitational 3-suiter hand-type to the Redouble (ie: 11-13 with 4441 or perhaps 5431 shape). This is shown by raisingthe suit Opener bids if they have hit one of your 4-card suits, or bidding 2NT if Opener has hit your shortage.
Pass commands Opener to redouble and is either
Weak with any 4432 distribution. With this hand-type Responder bids his lowest 4-card suit at the 2-level over the redouble and 4-card suits are bid upwards until at least a 4-3 fit is found.
Strong (Any hand that wants to defend 1NT redoubled goes this route and Responder simply passes over the redouble by Opener, forcing Opps to takeout or risk 1NT redoubled making)
[Optionally] Invitational with an 2-suiter, normally at least 5-5 in shape. Responder shows this hand-type by jumping to the 3-level in their lowest-ranking suit over the redouble. Opener passes (or raises) or bids the cheapest suit in which they would not accept an invitation to game. eg: in the sequence 1NT-(X)-No-(No)-XX-(No)-3♦-3♠-4♥ Responder shows an invitational 2-suiter with 3♦. Opener's 3♠ bid says "I prefer Spades to Diamonds, but have no game interest there, but I do have game interest in Hearts if that is your second suit"
[Optionally] Over the Redouble from Opener, bids from 3NT up to 4♥ are all transfers with game-going 2-suiters. These will probably be hands with fairly extreme distribution. Opener here breaks the transfer by skipping the transfer suit if Responder is transferring to their doubleton and Responder now passes or corrects to their second suit.
When 1NT is doubled, no scheme of responses is 100% foolproof (One weakness with INTRO is when Responder has a weak 1444 distribution with a singleton Spade and Opener has 4333 distribution with a 4-card Spade suit. This means playing in a 4-3 fit at the 3-level, almost certainly doubled and one alternative is to treat 1444 hands (ie: with a singleton Spade) as if they as were 2344 shape). INTRO does cater for most eventualities, however, and most importantly does give Responder a mechanism both to escape with a weak hand lacking a 5-card suit and to punish opponents when they have a hand capable of defending 1NT.
As stated above, INTRO itself doesn't really cater for 4th-in-hand penalty doubles, because Responder is in the protective position over such a double and therefore Pass is not really an option without giving up the main purposes behind INTRO.
PESKY (stands for Piglet ESKapologY LOL) turns this around by concentrating on Opener's actions over a 4th-in-hand double, but is fairly close to INTRO otherwise. So after 1NT-(No)-No-(X)-?? (where the Double is for penalties), Opener can assume that Responder
Doesn't have a hand worth an invitation
Doesn't have a 5-card or longer Major
Is unlikely to have a weak unbalanced hand with a 6-card Minor
...because if they had any of those three things, they would definitely be bidding over 1NT (which they didn't do).
Bids a 5-card Minor if they have one. Responder normally passes, but over 2♣, they may bid 2♦ with 5-card or longer Diamonds and no good tolerance for Clubs.
Redoubles with any MAXIMUM 4333 1NT Opener. Responder passes if they're happy with 1NTXX or bids their cheapest 4-card or longer suit and we play there.
Passes with any other hand. Now...
If Responder Redoubles, this is a transfer to Clubs and over 2♣ from Opener, Responder passes or corrects.
If Responder bids rather than redoubling, we wriggle upwards until we hit at least a 4-3 fit.
If Responder passes, we simply play in 1NTX.
So the meaning of Pass, Redouble and Bids by Opener is vaguely similar to those by Responder in an INTRO situation, although the continuations are slightly different and it is possible for us to play in 1NTX.
PESKY has the advantage over other conventions designed to cope with 4th-in-hand doubles that Opener's actions closely mirror Responder's actions in INTRO rather then being pretty different.
OCP System Change Log
A Change to PESKY (13-Feb-2022)
After 1NT-(No)-No-(X), Opener BIDS a 5-card Minor if they have one. A REDOUBLE shows any MAXIMUM 4333 and PASS shows any other hand. Over a Redouble by Opener Responder stands the Redouble or simply runs to a suit and we play there. Over a PASS by Opener, a Redouble by Responder is showing a 5-card or longer Minor and Opener bids 2♣, after which Responder passes or corrects. If Responder instead BIDS a 2-level suit, this is simply scrambling and we stop once we find a 4-3 or better fit. Responder can also simply PASS, in which case we play the hand in 1NTX.
Introduction of PESKY (26-May-2017)
PESKY is a suggestion for use after a 4th-in-hand penalty double of our 1NT Opening (ie: 1NT-(No)-No-(x)-??. PESKY concentrates on Opener's actions over the Double, and closely mirrors Responder's actions in INTRO. Opener is 2nd-in-hand over the penalty double and therefore a forcing pass is an option for them.
With PESKY, Opener bids with a 5-card Minor and redoubles with 4333, both showing lower to mid-range hands (Responder normally passes if Opener bids something, and wriggles upwards over a redouble). A Pass by Opener shows either a maximum hand or a lower to mid-range hand with 4432 distribution. Responder now redoubles with a hand just short of an invitation, and wriggles upwards if they can't afford to play in 1NTXX even opposite a maximum hand. If Responder redoubles, Opener passes with a maximum hand and wriggles upwards themselves with a 4432 hand not prepared to chance 1NTXX.
See the INTRO section of the 1NT Page for full details.
Eta After Stayman (07-Apr-2013)
In the interests of symmetry and consistency, the sequence ...1NT-2♣-2♠-3♥, which had been treated as natural and invitational in Hearts, will now be treated as Eta in Spades. This will mirror the sequence ...1NT-2♣-2♥-3♠, which was always Eta in Hearts.
Super-Accepts over 2-way 2♦ (07-Apr-2013)
Over the sequence 1NT-2♦, Opener always assumes Responder is transferring into Hearts. If they have a hand worth a super-accept of Hearts, they bid as follows:
With any 4432 shape and a 'worthless' doubleton (xx or Jx), they bid the suit below it (ignoring No Trumps and Hearts).
With 3433 Shape, they bid 3♥ instead of 2♥.
With any 4432 shape where the doubleton is not 'worthless', they bid 2NT rather than 2♥
This gives Responder a better idea how well the hand is fitting, and particularly how to proceed when they have less than 4-card Hearts themselves.