awm, on 2011-November-15, 19:29, said:
Some examples where 2/1 is better.
(1) Partner opens 1♠; I have a big balanced hand without a fit, say 2335.
(2) Partner opens 1♠ and I have a game force with 3-card support.
(3) Partner opens 1♠ and I have a game force with a ton of clubs.
(4) Partner opens 1♠ with a 5332 pattern, and I have a weak hand with a ton of clubs.
All of your examples can also be strengths for non-forcing 2/1s. In 1 Responder will bid 1NT as a relay. Then (in my version) Opener bids 2D showing hearts and Responder relays again with 2H. This has the same effect as 2NT in 2/1 (patterning out) but with even more space.
For 2 it would start the same way (1S - 1NT; 2D) then Responder has the choice of 3S for a cue auction or 2H to continue patterning out. Again, there seems to be no disadvantage to non-forcing 2/1s.
In 3 there is a potential disadvantage in that you get to ask Opener's shape but not directly show how good your clubs are. Of course you still discover whether there is a fit or not but you do lose the 3 level for advanced cues.
4 is of course a strength for non-forcing 2/1s. You can bid 2C getting your suit immediately into play and putting pressure on LHO since Opener will often pass with a minimum and 3+ clubs.
awm, on 2011-November-15, 19:29, said:
Here are some examples where 2/1 is worse:
(1) Partner opens 1♠. I have an invitational 2434 hand.
(2) Partner opens 1♥. I have a 3244 hand with minimum responding values.
(3) Partner opens 1♠. I have a 1426 ten-count.
(4) Partner opens 1♠, I have a 2443 hand with less than game values.
Note that most of the above advantages are on invitational hands. The relative frequency and importance of invites versus GF hands depends on how light you open and on the form of scoring!
One way around one of the issues in 1 is to open 1C on all balanced hands outside of the 1NT range. Then 1S - 1NT; 2m promises a 4 card suit. This is obviously not to everyone's taste though. The non-forcing 2/1 auction is here equivalent or even slightly worse, 1S - 1NT; 2C (min without 4 hearts) - 2NT. The upside is that if Opener is not minimum then we can continue in relays, in effect reverting to the GF auctions that were a system plus. That is the primary trade-off for non-forcing 2/1s - you sacrifice part-score clarity for a minmimum Opener opposite an invitational Responder to maximise effectiveness in game and slam auctions, as well as occasionally having a slight edge when Responder is very weak.
2 is another positive for non-forcing 2/1s. Responder simply bids 2C and we either play there or happily pass 2 of either red suit.
3 too, is no problem for nf 2/1. If 1S - 1NT; 2D (4 hearts), we rebid 3H if we still only want to invite, or more likely we take a stronger action as we are now probably worth game. If Opener is minimum and does not have hearts they will rebid 2C over which we can either pass if we are really poor or bid 3C which shows an invitational hand with 6 clubs. If Opener is maximum without 4 hearts they will make some relay response and we can find out everything we need to pick the right game.
And also with 4. The 10-11hcp hand would have started with a 1S relay. When partner shows hearts with 2D Responder just bids 3H to invite. The 8hcp hand bids 2D and raises 2H to 3H. Simple.
The above makes it look that the non-forcing 2/1s with relay approach is much better than 2/1. This is not really true and there are other examples I can provide that would show the reverse. I hope it does show that non-forcing 2/1s are competitive with standard 2/1 however. The basic concept is to try and minimise the 'bad' hands to those where Responder is invitational and Opener is minimum without hearts. On these hands you are worse off than either of the other systems though. As Adam says, how important this is to some extent dependant on other parts of the system. His analysis also shows why playing invitational-plus 2/1s with Fantunes one bids is such a bad idea.