When your partner opens in a major suit, playing
"2 over 1 forcing game", it is essential to the system that you respond 1NT for one round force.
Opener    Responder                        or        Opener   Responder
   1             1NT  <= forcing                          1            1NT  <= forcing
and the range of 1NT change from 6-9 to 5-11 hcp (high cards points)

However, many partnerships adopt an inverted response bidding for 1 and 1NT, after partner has opened 1, because this makes an improvement in the bidding. Without the inverted response, minimum hands (12-15 hcp) in 4=5=2=2 distribution like:
KJxx Q9xxx Ax Kx      or       AQxx J87xx Ax Qx
have no suitable bid after 1NT forcing because opener would have to bid 2 Clubs with 2 cards or rebid his 5 cards heart suit. 

Examples of 1nt inverted and forcing are: 
Opener Opponent Responder
   1         pass         1S  => is 1NT forcing (5-11 hcp) up to 4 cards in Spades;
   1H         pass         1NT => shows 5 or + cards in Spades and unlimited hcp.

After 1S from responder,  the opener of 1 Bids:
a) 1NT to show 4 cards in Spades (hand type 4=5-x-x) and minimum. If responder has 4 cards in Spades he bids 2 with 5-9 hcp and 3 with 10-11 hcp.
Example: Opener => KJxx Q9xxx Ax Kx  Responder => A9xx 10x Jxx Qxxx
Opener Responder
   1         1   Response denies 5 cards in spades and can have zero up to 4 cards so must be alerted (!)
   1NT        2   is a SIGN-OFF and must be alerted because it shows 4 cards in spades.

b) a minor suit at level 2 to show his second suit (can be 3 cards in 5-3-3-2)
Opener Responder      
    1         1         Opener  without 4 cards in other suit and without 6 cards must bid 2 with 3 cards  
 2minor                   in Clubs (a) or 2 with 3 cards in diamonds (b) (in case of 2 cards of Clubs)
Examples:  (a) A9 KJ10xx Qxx Kxx   or   (b) Kxx K9xxx AJx Kx

c) 2 with 6 or more cards 12-15 hcp and may or may not have unremarkable spades holding 
(with 8xxx AKJ1087 Ax x better to say 2 than 1NT after 1 from partner).
Opener  Responder
   1            1
   2 <= shows 6 (or 7) cards in h

d) 2S to show strong hand 18-20 HCP (hand type 4=5-x-x). If partner then bids a minor suit, with singleton we can pass, but with 3 cards we can go to game in his suit by either making an invitation or bidding directly 5 in his suit depending on our first controls (Aces) in majors.
Opener  Responder                      Opener  Responder
   1            1                                 1           1
   2 <= shows 18-20 hcp            2           3 <= shows 6 or 7 cards sign-off 5-7 hcp
                4=5-x-x                             ?
                                                       Opener with  AQxx KQJxx AQx x  must pass
                                                       Opener with  KQxx AKJxx Kx Kx   must pass
                                                       Opener with  KQxx AKJxx x AJx   bid 4
                                                       Opener with  AKxx AK9xx x Axx  bid 5 

e) 2NT to show strong hand 18-20 HCP, necessarily with 4 cards in other suit (hand type 5-4-x-x). This bid will distinguish a 5-5 distribution from a 5-4 distribution.
So after 2NT, the responder bids 3 asking for explanation of the suit.

If the Opener has opened 1 and he doesn't have 4 cards in spades, he bids:
3NT => having 4 cards in Clubs with 5-4-2-2;
3  => having 4 cards in Clubs with 5-4-3-1;
3  => having 4 cards in Diamonds with 5-4-2-2;
3  => having 4 cards in Diamond with 5-4-3-1;
4  => having 7 cards in Hearts, closed suit, with 18-20 hcp

If the Opener have open 1 then he bids:
3NT => having 4 cards in Clubs with 5-4-2-2;
3  => having 4 cards in Clubs with 5-4-3-1;
3  => having 4 cards in Hearts with 5-4-x-x;
3  => having 4 cards in Diamonds with 5-4-x-x;
4  => having 7 cards in Spades, closed suit, with 18-20 hcp

If responder bids a different suit, without asking 3, this is sign-off, so if he bids your singleton or doubleton you should pass, but if was your tripleton (or 4 cards in a tricolor hand) you can think in game and depending on your first controls in the majors and honor in support of your partner, you make a invitation or directly bid game or maybe a small slam!

f) Jump in another suit is 5-5-x-x with 18-20 hcp. The exception occurs in the rarely hands like 6-5-x-x where the 15-17 hcp are all concentrated in the 2 suits making strong suits with 4, 3 or 2 losers, because you cannot open in 2 having only 16-17 hcp even with 2 presumable losers!
Experience shows us numerous disasters when the opener of 2 Clubs having two long suits must make his second declaration at level 5 and his partner passes in his singleton having 4 cards in the other unbidded suit => 2 4 pass pass
What do you bid with x AKQ10x AKJ9xx x, 4NT or 5 or 5 or double? Do you make an agreement with your partner about that? And if your partner doubles 4 to show blank hand or 10xxx in Spades expecting to find you with 4 or 5 sure tricks, what will that outcome be?

g) Jump in the same suit (denying closed suit) is always a problem because some partnerships prefer to use the jump just in invitation cases, with 6 cards and 16-17 hcp (used more), and others use it in game forcing to show 18-20 hcp. Both ask for 2 cards support from partner, but the first case (invitation) needs also 8 hcp or more to continue. In both cases another suit from responder shows weak hand with 6 or 7 cards, so with singleton the Opener can pass even with maximum hand.
Note: It is important to assume that always another suit from responder is not forcing but just a invitation only in his suit (in misfit hand drop the bidding as soon as possible Terence Reese).

h) Jump in 3NT shows 18-20 hcp in hand type 5-3-3-2, but if the partnership use the jump rebid to show invitation with 16-17 hcp with 6 cards (used more) with 18-20 hcp and 6 solids cards in your suit you can also bid 3NT.

i) Jump to game in major suit tends to show 18-20 hcp with 6 or 7 cards in solid suit.

In the opener's second bid, after responder bids 1NT showing long suit in Spades (unless 5 cards), the opener bids:
j) 2S with 3 (or 4) cards in Spades and minimum hand 12-15(bad) hcp
k) 3S with 3 (or 4) cards in Spades and invitation hand 15(good)-17 hcp
l) 4S with 3 (or 4) cards in Spades and strong hand with 18-20 HCP (4-5-2-2 or 3-5-3(2)-2(3)
m) 4 in a minor suit at level 4 (splinter) showing a singleton with 4 cards in Spades in strong hand 18-20 hcp 4=5=3(1)=1(3) or 4=6=2(1)=1(2) 
n) jump to a minor suit, forcing game, in principle 5-5-x-x, and then bidding 4 to show 3=5=4(1)=1(4) or 3=5=5(0)=0(5) in a hand of 18-20 hcp. The singleton or void is marked by exclusion.

In hands with 16-17 hcp, opener after 1NT must bid like having 12-15 hcp in first round and bid again if he has chance:
A 1NT response after 1 opening or 1 response after 1 opening shows 5-11 hcp and can be done with many types of hand, so the Opener must be careful in his second bid and prudent assuming  partner with minimum range hand 5-7 hcp in principle. So he needs to bid like having 12-15 hcp and not 16-17 hcp, because a jump is reserved for a strong hand with 18-20 hcp.

Partner could have a middle range hcp (8-9) or a maximum range hcp (10-11) hand.
If your partner has a maximum range (10-11 hcp) after your bid, he must bid again, normally NT or jumping to his long suit or jump to your suit to show 3 cards support or making a simple raise in your other suit. In all these cases you can go to game without problem.

It is important to emphasize  the substantial difference in support at level 3 between SAYC and 2/1:
        In SAYC                                                         In 2/1                                             In 2/1
Opener  Responder                                   Opener  Responder                     Opener  Responder
   1           3  <= 10-11 hcp                     1         3 <=10-11 hcp            1          1 (5-11 hcp)
   1           3  <= 10-11 hcp                     1         3 <=10-11 hcp            2minor   3 10-11 hcp
Opener doesn't Know if 3 or 4 cards    Opener Knows 4 cards support    Opener knows 3 cards support

If your partner bids other suit, it is a sign-off for convention, and if you are with 12-15 hcp you must pass, but having 16-17 and 5-4-2-2 hand or 5-4-3-1 and no singleton in his suit, you need to bid again, bidding NT at level or supporting him at level 3 with 3 cards. Your partner will know that you are 16-17 hcp and then he can decide if he should go to game, or pass or rebid his long suit definitely a sign-off.

Open 1NT with 5-3-3-2 having 5 card major and stopper in 3 suits only if you have 16-17 hcp and with  15 hcp open in your major suit. This is a good choice to follow and simplify your rebid after the response of 1NT from your partner. The transfer and Stayman (Puppet) mechanisms are better than in 1NT response, and you can show your hand quickly.
                                                                                                                      revised by Allison Schnabel
                                                                                                                                          from Arizona - mar/2004