NT Defence

♣  ♥  ♠ ♦

Opponents opened 1NT 

A 1NT opening is usually a balanced hand, although some alert a  possible singleton, with a specific HCP range.
However various ranges are common, such as 12-14 (ACOL), 13-15, 14-16, 15-17(SAYC) are all encountered.
Some play a variable NT, be alert, which is stronger when vulnerable. e.g.12-14 non-vulnerable & 15-17 when vulnerable.
Beta prefers a 12-14 Balanced NT see our 1NT_opening. Its worthwhile to read this before reading our defence to an opponents..

Our defence in 2nd position:

CDH                  Clubs, Diamonds, Hearts .... This is our long tested  defence at the 2-level....since 1996
Bidding any of CDH shows the suit bid and also promises a higher ranking suit too - As a 2nd suit exists when partner cannot stand the bid suit, holding 2 or less cards, then he reaches out for the 2nd suit - a process called a 'wriggle' - bidding tthe next higher suit of at least 3 cards, headed by A|K|Q or a 3 card suit until a satisfactory fit is found. At worst this will be a 4-3fit  hopefully a 4-4 or 5-3 fi is discovered. These suits may be 4 card but 5-4 is safer especially when you are vulnerable or following a strong NT opening.
A 2♠ Overcall is natural -  showing at least a quality 5 card Spades suit.

Rubinsohl            Rubinsohl (our usage is actually somewhat closer to Grubinsohl) it provides a defence against a 1NT opening bid  (also against weak 2's). All bids: 2N thru 3♥ inclusive are each transfers to 3♣ through 3♠ inclusively.          
Rubinsohl is our defence at the 3-level and requires a good 6+ card suit. Each bid is a transfer which partner must complete. If the Rubinsohl bidder has a 6-4 shape, the 4 card 2nd suit can be shown, if prepared to play at the 4 level, in either suit. Responder can except the 2nd suit with 4 card support or revert to the first suit that you were required to bid. 

Double                A double over a 1NT opening suggests penalties usually with a good 15+ HCP over an ACOL weak NT (Probably based on quick tricks and on establishing & running his long suit ♣|♦|♥) but it will categorically deny holding 2suits or a 5+ card Spade suit. - however partner may take out into his own 5-6+ card suit if available. Allowing 1NT to be played and defeated may not compensate for a missed game or even a 3 or 4 level partscore!   The danger of issueing a double in 2nd position is not yet knowing the values held by the your yet to bid LHO! if he can issue a classical redouble this can spell doom for the defending side.

 Bear in mind an ACOL 1NT of 12-14HCP - leaves 26-28HCP around the table....Responder competing with distributional hands is a must. You may regret it on oiccasions BUT on many others you will make sizable gains. Responder with +/-7 high card strength may leave the Double and defend.

2♠                      5+ Cards - competitive.

PASS                 There is no shame in PASSING! do not compete for the sake of it.

Defence in the protective, 4th position:

Sitting under the NT bidder presents a greater interventional risk than  in 2nd position. However if one thinks about it there is a risk in 2nd position too (e.g. opener's partner's strength is still unknown!) While in 4th position opener's partner has or has not bid!
However we may nevertheless intervene with a quality Major suit - at least 5 cards.
We can also intervene with CDH too:  Slightly modified in the protective position -
Lastly we may use a protective double with a flattish hand. This is NOT a penalty double - it  indicates a willingness to compete and suggests some support in all unbid suits and his partner can choose a preferred suit. However partner may decide to convert it into a penalty double by passing. In making this decision one should realise that his partner is suggesting to compete and may well not hold values to issue a penalty double. One should protect this ability to compete against a weak 1NT opening e.g. 1N-P-2D!-*-P-?  "Partner: Take Out in any suit bar Hearts"
While a sequence like:  1N-P-P-* suggests its a 50-50 situation and has thrown the decision "whether to bid or to defend over to you". PASS converts to penalties or bid a suit to play in it...

All these protective bids are made on a presumption that points are divided at least  20-20 or better for you. That is expecting partner to hold the balance of 20HCP minimum.
In fact a protective bid is based on the presumed combined values of the defence. Getting this right can mean nuch success!
This makes sense as declarer has many advantages and more than likely to succeed in 1NT giving you a -90 score & sometimes -120. To compete at the 2-level when non-vulnerable could make +90 or even +110 and if it is defeated -50, sometimes -100. On balance one should therefore compete from 8+HCP! Partner would not have intervened with a flat hand or a simple 5 card suit (except in Spades) with 10-12HCP. However in CDH he would intervene with 2 suits...and double with (13)14+HCP.
Therefore,with CDH allowing intervention holding 2 suits, failure to intervene in 2nd position suggests a single suited hand which playing in it suggests a favourable outcome if partner issues a protective double.. The risk of 1 down is higher if partner has a flattish hand. In practice expect success on at least a 3:2 basis more likely even 2:1 - over many deals that should show a very healthy profit.
So although ACOL requires a double in protective position to be 15HCP, I  disagree.  My basis is "what i don't have, my  partner has" Furthermore if this is a truism we may conclude if i have less my partner has more and he sits over the opener!
In 2nd position i double with 13, the average of opener's range if its 12-14 range Not ACOL's 14-15.

Rubinsohl is NOT available in the protective position. With a substantial suit 6+ cards one can bid 3 of a suit in the protective position. This puts the 1NT bidder on lead (this is an advantage to the defence).
Note: This highlights the difficulty of doubling in 2nd position where one then has then to lead up to the NT hand: which can often lose a trick for the defence.

Lead Directing Doubles

This position gives good opportunities to indicate a preferred lead to your partner when the final contract is in NT's by your LHO:-

Doubling an artificial response bid made by your RHO opponent is a lead directing double - typical possibiliyies are 2C stayman, transfer bids: 2D, 2H, 2S (for clubs)  where a double requests the suit  of the artificial bid made by dummy to be led by your partner against a final NT contract.

Another useful idea is when your RHO has indicated  2 suits: Consider this example:

1N(LHO)-P-2H(transfer to spades by RHO)-P(you)-2N(LHO)-P-3C(RHO)-P(you)-3N-P- P-*- all PASS

This final penalty double is also a lead directing double in the first suit bid or implied by your RHO - in this case Hearts. [Some people advocate it to be a  request in the 2nd suit (clubs here) - but I strongly diagree as this is more likely to be a natural choice anyway]

Other Lead directives

(i)    If your partner has called a suit during the bidding this is usually taken to be lead demanding. The onus on your partner is not to make average looking overcalls if you wish to use this as a directive:
             Ex:  1N-P-2C-2H-3N-P-P-P    ............. Lead Hearts 
(ii)    Your partner calls a suit in the auction - again you should probably consider to lead it - but certainly lead it if partner also doubles the final 3N call by declarer.