Weak NT structure

The opening 1NT bid

The requirements are simple: 12-14 HCP and a balanced, or semi-balanced, hand. 1NT promises at least 2 cards in every suit, and denies a 5-card major. This implies that a 7222 shape is acceptable. The 1NT bid says nothing about stoppers.

In third seat it's permissible to shade the lower end to 11 HCP.

Opening 1 of a minor

This is one of the most significant aspects of the 12-14 NT structure. Opening 1 of a minor denies holding a balanced 12-14 HCP, and therefore implies one of two possible hand types:

15+ balanced. This includes hands that would open a 15-17 1NT, and also hands that are too strong for that (18-19 HCP). For 18-19 point hands, it makes no difference whether you play 12-14 NT or 15-17.
A distributional hand containing a singleton or void.

Responses to the opening 1NT bid

All of the conventional responses to 1NT work fine with the weak NT, with the point ranges adjusted accordingly. One small, but important, difference is that you generally need only 24-25 HCP in the combined hands to make 3NT: when the points are about equally divided the hand tends to play well. Therefore you should make an invitational raise in NT with 11 HCP points and force to game with 12+ HCP.

Garbage Stayman becomes a particularly worthwhile convention, especially when you hold a weak hand opposite the 1NT opener.

If you like to play that a jump to 3 of a minor shows a broken 6-card suit and invites 3NT, your hand should also have one outside trick opposite the 12-14 NT [this is my best guess as to how this bid should work].

Responses to 1 of a minor

The rule here is: Assume, until proven otherwise, that your partner has a balanced 15-17 count. This has several consequences:

When partner opens one of a minor and you have a weak hand (less than 5), poor support for opener's suit, and a decent higher-ranking 5-card suit, try to scrape up a bid. You will not get too high if partner continues in a disciplined manner (see Opener's Rebids section).
1NT over 1 of a minor shows 5-8 HCP. It is a hand that would not have made an invitational bid opposite a 15-17 NT.
2NT over 1 of a minor shows 9-12 HCP. It is a matter for partnership discussion whether this should be treated as forcing, since if opener has shape he should tend to pull this bid to a suit contract, and if opener has 15+ he will always make another bid.
3NT over 1 of a minor shows 13-15 HCP.

Rebids after opening 1 of a minor

The rule here is (I'm repeating myself): Partner is assuming that you have a balanced 15-17 count unless you do something to prove otherwise. That something is to bid, or raise, a suit.

For example:

You	Partner
1	1
2

2 is alertable (although not everyone does) and shows one of two hand types: (1) a balanced 15-17, or (2) a distributional hand containing a singleton or void.

You	Partner
1	1
2

2 is alertable. It shows a hand containing a singleton or void. Its strength is limited only by opener's failure to jump shift.

You	Partner
1	1
2

2 is alertable since it shows a hand containing a singleton or void. Just as in standard methods, it's a reverse promising extra values and is forcing for one round (except at the San Mateo Bridge Club).

You	Partner
1	1
2

2 is alertable since it shows a hand containing a singleton or void, and typically 6+ diamonds with a minimum in high cards.

You	Partner
1	1
2NT

2NT is not alertable. It shows 18-19 HCP balanced, just like the standard 15-17 NT system.

You	Partner
1	1
1NT

1NT is alertable. It shows 15-17 HCP balanced, and it could conceal a 4-card spade suit, 3-card heart support, or both. With invitational values or better (only 9+ in this system), responder can bid New Minor Forcing to discover the best fit. With a bust hand he can pull to 2 of opener's minor or 2 of his own major (holding a 5+ card suit).

You	Partner
1	1
3

Probably alertable but I'm not sure. It shows a good 15 to a bad 17 HCP with shape (singleton or void), or 18-19 balanced, and it promises 4 card support. Here is why you don't get into trouble by bidding aggressively with poor hands opposite partner's 1 of a minor: opener has this very accurate bid available to invite game. All it takes to accept is a 7 count since partner is promising 18+ support points. This bid will rarely be passed but it is an important part of the system. It also means that an immediate jump to 4 of responder's major guarantees 20+ support points.

Runouts after a direct seat double

If opener's LHO doubles a 12-14 NT for penalty, you must have a method to find the most playable contract. There are two that I know of.

Hiyashi

After 1NT - X:

2 of any suit shows a 2-suited hand consisting of the suit named plus the next higher (2 shows spades and clubs). Opener passes or corrects.
Redouble shows a single-suited hand, and forces opener to bid 2 (assuming the next player passes).
Pass forces opener to redouble, and now responder can:
- Sit for the redouble with a good hand;
- Bid a suit, which again shows a 2-suited hand consisting of the suit bid plus the non-touching suit. (2 shows clubs and hearts, 2 shows diamonds and spades). Opener will pass or correct.

DONT

After 1NT - X:

2 of any suit (except spades) shows a two-suited hand consisting of the suit named plus a higher-ranking one. Opener passes or shows a preference.
Redouble shows a single-suited hand, and forces opener to bid 2 (assuming the next player passes).
2 is to play.
Pass forces opener to redouble. Responder will usually pass for penalty, but might make a conventional bid (Stayman and transfers are on if the redouble gets back to responder).

Competitive auctions after 1 of a minor

The rule typically doesn't change when the opponents get into the action: Opener is assumed to have a balanced 15-17 count unless he does something to prove otherwise.

The only time the rule is suspended is when the opponents have competed beyond 3 of opener's minor by the time he has a chance to bid again.

Therefore:

Opener	LHO	CHO	RHO
1	1	Pass	2
Pass

Opener's pass is alertable and shows a balanced 15-17 HCP. Interesting.

Opener	LHO	CHO	RHO
1	1	Pass	2
3

Opener's 3 is alertable and shows a hand containing a singleton or void.

Opener	LHO	CHO	RHO
1	1	Pass	2
2

Opener's 2 is alertable and shows a hand containing a singleton or void.

Opener	LHO	CHO	RHO
1	1	Pass	Pass
Double

Opener's double is not alertable. He has not yet defined his hand type.

There are lots of other possible auctions but the principle is the same. This leads to two very important rules:

Don't open 1 of a minor with a junky rule-of-20 hand that has a lot of minor-suit cards. You may get stuck for a rebid if the opponents get into the auction.
Don't open 1 on a minimal 1-3-4-5 or 3-1-4-5 hand, say with 11 HCP and a mediocre club suit. You might try 1 but I will bet you won't get to the right spot. Pass and hope for the best. The same considerations apply to 1-4-4-4 or 4-1-4-4 hands.