F or NF?
Due to (constant) student request, I have given in and written about this annoyingly tricky topic.
How hard can it be? "Just tell us Larry, which bids are forcing and which are not."
It is very hard. It requires study, memory and sometimes partnership agreement (PA).
F versus GF
In this series, it is important to recognize the difference between Forcing (F) and Game Forcing (GF). "Forcing" (F) means your partner "cannot" pass the bid. If he does pass, he does so at his own peril. I might do it once a decade. Suppose my partner opens and I dredge up a response with a very weak shapely hand. He makes a forcing rebid but I bail out with a pass. I'd better be right! "Game Forcing" (GF) means neither partner can pass below game.
In this 5part series, I plan to break it down as follows:
 Our opening bid, response and opener's rebid (with no interference)
 The fourth bid and beyond of our auction (with no interference)
 After Overcalls (by us or the opponents)
 Auctions with Doubles
 Other/Conventions
Last month, we looked at #1)  Our opening bid, response and opener's rebid.
In this article, we address #2:
2) The fourth bid and beyond of our auction (with no interference)
Again, we assume no interference and that we are not in a 2/1 GF auction (where obviously we are forced until game is reached). Let's start with auctions where our response is 1ofasuit.
Opener Rebids on the 1 level in a new suit:
After 1 of a suit1 of a suit, what if opener bids a 3rd suit? Let's start where the third bid is still on the 1level. So, we are looking at these auctions: 111, 111 and 111. In all cases, responder's bid of the 4th suit is forcing. So, in the auctions above, it would be 1, 2 and 2 respectively (though not all partnerships agree on 1111). For more on fourthsuit forcing, click the link. If responder bids anything but the 4th suit, it is not forcing. His bids of 1NT, 2NT or 3NT can end the auction. If he repeats his suit or raises opener's suit (on any level) it can end the auction.
Opener Rebids on the 2 level in a suit:
What if opener's rebid was a new suit on the twolevel? If opener reverses or jumpshifts, that is forcing (other than a jump rebid of 2NT which is 1819 balanced). Any other new suit bid on the twolevel by opener is NF (for example, 112, 112, 112 or 112). So, what if responder bids over any of these NF rebids? If he repeats his own suit, it is weak, NF. If he jump rebids his suit to the 3level, it is invitational (NF). If he jump rebids to game, that is also NF. If he raises opener's suit to the 3level it is NF. If he bids 2NT it is invitational (NF). Jumping to 3NT is obviously NF. Really, the only way for responder to force is to use 4th suit forcing.
What if opener's rebid is 2 of his own suit? If opener rebids his suit on the twolevel (after a onelevel response), that shows a 6card (almost always) suit and is surely NF. So, we are talking about these starts: , 11 any2, 11 any2, 11 any2 and 11 any2. After any of those rebids, responder has to bid a new suit to force. Anything else by responder (notrump, repeating his suit, or raising opener's suit) is NF.
Note: If the auction begins 1 any1NT2 of a suit, there isn't really any forcing bid the responder can make. If he bids a new suit, it is NF. If he raises either of opener's suits, it is NF. If he bids notrump, it is NF. The only forcing bid is "the impossible 2." What is this? It starts 1 any1NT2any2. Responder can't have 4 spades (since he didn't bid 1), so 2 is forcing. Typically it is some maximum with support for opener's second suit.
Opener Rebids on the 3 level in a suit:
What if opener's rebid was on the threelevel? Again, opener's jumpshift is F (GF). Opener's jump rebid (113, for example) is NF. Opener's jump raise (113, for example) is also NF. What if responder bids again after one of those NF rebids? Anything responder does is GF! So, for example, 1133 is forcing.
Opener Rebids Notrump:
What about if the auction starts 1 of a suit1 of a suit1NT? These are the auctions: 111NT, 11MAJ1NT, 11MAJ1NT, 111NT. In all cases, opener is showing 1214 balanced.
Raises of notrump (even 4NT) are natural and not forcing. Repeating a previously bid suit (even jumping) is not forcing. What about bidding a new suit? Now we get into PA. A lot depends on whether or not the partnership uses newminor forcing. So, 111NT2 would not be forcing if newminor forcing were available. Furthermore, jumps in new suits (111NT3 might be played as weak (since with a decent hand, responder would start with newminor forcing). This complex area is all a matter of PA.
If opener rebids 2NT, newminor is also a possibility. Unless using a special convention (such as Wolff signoff), any suit bid on the three level by responder should be forcing (for example, 112NT3).
Opener's Rebid is none of the above:
If opener jumpshifts, the partnership is forced to game (so anything by responder below game is forcing).
If opener reverses, it is not GF, but is forcing one round. Now, we are back to PA and you can read more about reverses by clicking the link.
Resonder's second bid:
If opener has rebid 1NT, responder's new suit (unless conventional) would be NF. Example: 111NT2 = NF
If responder's first bid was 1NT, his next bid (unless conventional) won't be forcing. Example: 11NT23=NF
If responder bids the 4th suit, it is artificial and GF. Example: 1122
If responder bids a new suit (and his original response was not 1NT), it is forcing: Example: 1123
I did warn you that this F or NF topic is not easy.
In the next article, we explore auctions with overcalls.
QUIZ
Is the last bid in this auction F or NF (no interference)?
Opener 
RESPONDER 
OPENER's REBID 
RESPONDER'S REBID 
A) 1

1

2

2

B) 1

1

1

3

C) 1

1

2

3

D) 1

1

1

2

E) 1

1

1NT 
3

F) 1

1NT 
2

2

G) 1

1

1

3

H) 1

1

1

2NT 
I) 1

1

2

3

J) 1

1

2NT 
3

K) 1

1

1NT 
4

L) 1

1

3

3

Each answer can be explained by carefully looking at the detail above.
A) F
B) NF
C) NF
D) F
E) NF
F) NF
G) NF
H) NF
I) NF
J) F
K) NF
L) F