Just for interest, here are some deals which slipped through where there was a suit without cover in the 1NT opener. 3NT is then out of the question but does 4H become more attractive or not? I have made the 1NT opener East every time and not put in any vulnerability conditions, which may well be of importance in the bidding when NS might consider bidding over either 1NT or the 2NT response. I leave the reader to decide what might happen under different vulnerabilities.
Deal 1:
When opener shows a club weakness, 4H must surely have many chances of making. In this deal, only the QC lead and a low diamond switch would defeat 4H if declarer falls for North’s clever switch. If declarer goes up with the DK in that scenario, an overtrick. But in reality at the table, NS will continue clubs for three rounds and 4H is then a claim. Note that when West asks with 2NT and decides on a final contract, NS don’t know very much about West’s shape or strength.
Deal 2
This time there are an inevitable four losers with the defence having little chance of messing up, but 4H is still worth bidding with the hand that West has.
Deal 3
This time we might get some bidding by North. 1NT from East, 2NT from West, and 3S from North. Would North bid 4S if West jumps to 4H without asking the question with 2NT? I would, but much could depend on vulnerability. But let’s say West bids 2NT and North 3S. How do we respond in the opener’s seat? With all suits stopped, we bid 3NT unless we have very good spades (this East hand comes close) in which case we double. With no spade cover we pass.
Deal 4
This deal could be interesting at the table. Depending on the vulnerability, even a jump to 4H might induce North to bid 4S, guessing that South has a singleton heart. East would probably double and West, with no defence, might take the push to 5H. Anything could happen whatever the bidding, so let us not conjecture further. What I will do now, though, is to suggest what to do as the 1NT opener when, after 2NT from partner, the next opponent comes in with a bid at the three level. We have a simple answer to that, as follows:
Over 3C, PASS shows no club cover, 3D, 3H, 3S no cover there, and 3NT cover in every suit.
Over 3D, PASS shows no diamond cover, 3H, 3S no cover there, and 3NT cover in every suit.
Over 3H, PASS shows no heart cover, 3S no spade cover, 3NT cover in all suits, and DOUBLE shows heart and spade (by inference) cover but no guarantee about either minor.
Over 3S, PASS shows no spade cover, DOUBLE shows spade cover but no guarantee about any other suit, and 3NT shows cover in all suits.
If either opponent jumps to the four level or higher, common sense and good judgement is necessary, but we have plenty of that, don’t we?