Responding to strong two no-trump bids  Stuart McPhee. 12/12/99. Perhaps surprisingly, the first bridge convention that my early tutors persuaded me to play was the Flint convention.
The idea was that you bid 3 diamonds over partners two no trump opener as a transfer to three hearts. Usually, you would then pass three hearts or maybe bid three spades - which was also to be passed. Strangely, advocates of the convention thought that it was wonderful to be able to alight delicately at the three level. The spin off advantages of getting the contract played by the strong hand did not seem to attract attention. Of course, like all conventions it was abused and the strong hand often found an excuse to bid game.

Auctions like this were common:
 
 
NORTH EAST SOUTH WEST
  Pass 2 No trumps Pass
3 Diamonds Pass 4 hearts Pass

South might hold something like:

ªKQ6 ©A643 ¨AQ7 §AK3.   Opposite something like:  ª3 ©J98752 ¨963 §654. This would sometimes make game and sometimes not. If you failed to make ten tricks you could expect a stern lecture with the words "lack of discipline" and "why are we bothering to play Flint" figuring prominently.  I can also remember thinking how elegant the convention was, because if responder continued 3 No-trumps after 3 Diamonds (on those rare occasions when partner did dutifully bid 3 hearts) then this showed a genuine 3 Diamond response so you did not even lose any descriptive bid. 

These days bidding theory has progressed and our systems for responding for strong two no-trump bids are much more sophisticated - although the fact there are still many options indicates that none are perfect. One of the reasons why it is important to have a sensible system is that two no trump sequences are much more common now. This is partly because of the widespread use of weak twos, multis and other distributional two bids. If you overcall these bids with two no trumps showing some sort of strong flat hand then it seems sensible to treat it as if it were a two no trump opener.

So what features need to be covered in your responses to 2 NT?  I would summarise then as follows:
1. You need to be able to discover whether the 2NT bidder has a five card major suit.
2. You need to be able to discover whether the 2NT bidder has a four card major suit (ie seeking a 4-4 fit).
3. In both the above situations, you want the 2NT bidder to end up as declarer (even if your partner is .... - no I won't mention names!).
4. You may want to be able to investigate minor suit fits.
5. You may want to allow responder to show hands with exactly 5 spades and 4 hearts.
6. You want to ensure that the 2NT bidder is declarer when responder holds a long maor suit.

The following scheme satisfies all the above:

    3C = major suit enquiry
                Then 3D = at least one 4 card major
                    (after this responder then bids major he does NOT hold, and bids 4D with both majors)
                3H = no major (at most 3 hearts and 3 spades)
                    ( after this responder then bids 3S with 5 spades (+ 4 hearts, which prevented transfer)
                3S = 5 spades
                3NT = 5 hearts
                      (responder then bids 4D to force opener into 4H)
     3D/H are red suit transfers, with continuations down to partnership agreement.
     3S  = minor suit enquiry see below.
     3NT = natural!
4C    = GERBER

Note the reversing of the normal meaning of the 3H and 3NT response. Although this is a memory worry, it does give responder a chance of showing those troublesome 5-4 major hands.

You can devise your own meanings for four level responses. If you want something simple then they can be Gerber and natural. If you want something obscure and complicated then look at Rosenkranz -  The Bidders Game or email me.

You also have several options after  2NT – 3S  (Minor suit enquiry)

You could try something like this:
    3NT denies a four card minor
            Then 4C/D single suited with slam interest  (>4NT sign off)natural
            or 4H/S = shortage with both minors (at least 5-4) >4NT = sign-off
     4C/D natural
     4H/S cue bids with both minors.
.
In subsequent auction, 4NT is RKCB or Blackwood (0 or 3, 1 or 4, 2), depending upon whether trump suit is identified.