The Case for the Weak NT: Advantages of opening 1NT generally

The limited strength and distribution of a 1NT opening allows auctions that are less revealing to opponents compared to opening one of a minor. The contracts reached after a 1NT opening are therefore more likely to make. You'd rather bid 1NT-3NT than 1D-1S-1NT-3NT because you have a better chance of a favourable opening lead and the subsequent defence will be more difficult as well.

The weak notrump occurs more often so it gains the most from this.

When the hand belongs to the opponents, it is more difficult and dangerous for them to enter the auction when you open 1NT, compared to opening one of a minor:

• They have to start bidding at the two level
• They have the additional objective of trying to get a penalty, not just bidding to their own best contract
• If they pass with a decent hand, there may well be no second opportunity
• Whether your partner passes or bids, there is still no safety for them as your side can have almost game values and no fit
• The limited shape and strength of the 1NT opener makes it easier for you to judge when to double them
• The 1NT opener has not shown a suit so takeout doubles and cue bids are not available for them
• Auctions over opponents 1NT are generally not discussed and understood as well as auctions over their suit opening.

Over a strong notrump, finding your best partscore is the overriding objective and methods can be tailored just to do that. Defences such as DONT, Meckwell and Lionel are all designed to do this but you'll find it hard to extract a penalty or bid constructively towards game if you try using these against a weak notrump.

There's no real solution to this - bidding accurately over an opponent's weak notrump is just extremely difficult. Here are a couple of examples:

♠	xx		♠	xx		♠	KQxx
♥	KQJ10xx	or	♥	KQJ10xx	opposite	♥	xx
♦	Qxx		♦	AQx		♦	Kxx
♣	xx		♣	xx		♣	Axxx

If West has the first hand, 1NT could well make if he passes and 3H is too high if he bids and partner invites. On the second hand, E/W have a game in hearts. After a 1C opening, both hands are easy to bid depending on whether your jump overcalls are weak or intermediate. Starting a level higher after a 1NT opening, there isn't enough space to do everything.

♠	AQxx		♠	Kxxxx		♠	xx
♥	x	opposite	♥	Qxxx	or	♥	QJxxx
♦	KJxxx		♦	Qx		♦	xx
♣	Qxx		♣	Kx		♣	Kxxx

Let's say E/W have a bid showing spades and another suit. West will be reluctant to bid over 1NT, especially vulnerable, in case East has the second hand and he concedes a significant penalty against a part score. Of course, if East has the first hand he will miss an easy game. There is no such problem after a minor suit opening.

♠	AQxx		♠	Kxx		♠	xxx
♥	KJx	opposite	♥	Qxxx	or	♥	xxxx
♦	AQJx		♦	xx		♦	xx
♣	xx		♣	QJxx		♣	QJxx

If West doubles, his partner with the second hand will have no choice but to pass and concede something like 380 (or 960 if they redouble). Obviously if he passes and partner has the first hand, E/W will miss either a game or a sizeable penalty. If West doubles a 1C opening for takeout opposite the second hand, E/W will get no higher than 1H or 1S, probably down one undoubled.