The Weak No Trump (12-14)

Introduction: In standard 1NT shows 15-17. With this range generally the partnership is safe at 1NT or 2 of a suit. However the 1NT opening does not come up all that often. In any case, most people try to disrupt the partnership's bidding after opening 1NT. For the most part such disruptions are successful since rarely is the hand theirs when an opponent's hand holds 15-17. 

On the flip side of the standard 1NT is the weak no trump showing 12-14 (a very good 11 could also be opened). Although it is not as "safe" as the strong 1NT, the weak no trump has some merits: 

1. it occurs much more often and if partner does not have enough for game and does not have a suit, then the partnership would have probably ended at 1NT anyway. 
   
2. the opponents are less likely to know what to lead. This is especially true when the bidding goes 1NT-3NT. At other tables the bidding may have proceeded: 1C-1H, 1NT-3NT giving the opponents reasons for not leading a suit.
   
3. when partner has absolutely nothing, it is likely the opponents have a partscore or even game in their hands. 
   
4. people are more apt to bid over a weak no trump opening. When they do so indiscriminantly their side is likely to get a bad score since their partners are usually screwed when they hold a good hand. (Against a weak no trump, the opponents are just as likely to have the balance of the points--- or even game--- as the opening side, so disruptiveovercalls are disruptive--- to the opening side). 

The weak no trump hand:
It is a balanced hand of 12-14 hcp. It denies a void, singleton, and a 5 card major. A good 11 point hand could also be opened 1NT if it has a good 2 1/2 tricks. 1NT could also be opened on a bad 15 (ie some points are duplicated--- like QJ doubleton). 

Responding:
Responder generally knows where the hand should be played. With 0-6 hcp, responder should usually run to a suit contract--- unless responder is also balanced--- in that case he may decide to just have partner "stick it out" at 1NT. With 7-10 responder knows that 1NT is safe and usually does not run unless he has a 5 card 
major. With 11 responder invites game (except in a minor, of course). With 12+ responder knows game is there and should make sure the partnership is there.

I prefer to use 4 suit transfers myself, but any other response structure is fine.