Here are three bidding sequences which you may not have discussed with partner.  (I think I have, but no doubt a comment will be added to this post if I am wrong!).  All three sequences involve partner's 1NT.  What, you have worked out all the sequences involving 1NT, including those when the opposition intervenes?  We thought so, too.

1.  As South, you hold something like ♠J74 ♥KJ985 ♦109  ♣J72, and the bidding goes like this:


  

West
Partner
East
You
  
  

1♦
1NT
2♣
?
  


Without East's pesky 2♣ bid, you would have bid a transfer 2♦, as you and partner play "system on" opposite the strong 1NT overcall.  But what does 2♦ mean now?  Should you bid 2♥, because you are now in a natural situation, or even a Lebensohl situation?  But supposing partner takes this as a transfer and bids 2♠?  Oops.  Conversely, if you bid 2♦ meaning it as a transfer, what happens if he doesn't see it that way and passes (particularly as the opening 1♦ might have been short diamonds)?  Double oops.  At the table I cravenly passed, not wanting to risk a possibly expensive misunderstanding.  Afterwards, we decided that 2♦ would have been transfer - "system on rules OK".

2.  What about this?


  

West
Partner
East
You
  
  


1NT
Dbl
Pass
  

Pass
Redbl
Pass
?
  


It doesn't much matter what you've got - the question is, why is partner suddenly redoubling?  He doesn't know if you have a lousy hand or one which is going to help 1NT make with overtricks.  Most people play redouble by South as the start of a rescue wriggle, or a Stayman substitute, or as transfer to 2♣.  So South was unable to redouble for business, as it were.  So what is North now trying to convey?
Well, it seems to me that if he has opened 1NT with a long (6+) minor, he could now bid it.  So redouble is likely to mean "I have a good five card suit; run if you want to".  Now if South judges that 1NT is going to make, he can pass, and otherwise he bids 2♣ which North either leaves or converts to his "good five card suit".

3.  And finally:


  

West
You
East
Partner
  
  


1NT
Dbl
Pass
  

2♥
Pass
Pass
2NT
  


When this cropped up some time ago, my partner meant 2NT as natural: "I think we can make 2NT pard, let's not sell out in 2♥". But I thought 2NT meant "I've got two places to play" so bid something or other - can't remember what after all this time.  Whatever it was, it wasn't a success.
Since then, partner & I have agreed that 2NT is almost always not natural, regardless of the sequence in which it crops up.  So if the same bidding occurred tomorrow night, I hope we would both remember that 2NT in this kind of sequence means "I have two places to play, partner, let's agree on of them and not sell out to 2♥".


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