In Keri a 2C response forces opener to bid 2D. There are then three basic sequences which take care of the hands which would have bid Stayman in standard methods:
- 1NT : 2C , 2D : 2M shows an invitational hand with 4 or 5 cards in the major.
- 1NT : 2D , 2H : 2S shows an invitational hand with both majors. (The 2D response in Keri is a transfer to hearts, but differs from a standard transfer in that in can have only 4 hearts if it is invitational with both majors.)
- 1NT : 2C , 2D : 2NT shows a game-forcing hand.
The first of these sequences is the one we need to focus on, since it is the only really fundamental part of the system. (There are variants of Keri which deal with game-forcing hands differently to the "book" version above.)
The idea is that after 1NT : 2C , 2D : 2M opener will pass with a minimum hand with 3 or 4 cards in the major. Thus a big advantage of Keri is being able to stop in 2M - particularly when this is a 4-4 fit (where a Stayman auction would have reached 3M after 1NT : 2C , 2M : 3M) or a 5-3 fit. There are plenty of examples of this in the book. The disadvantages, on the other hand, are not so clearly spelt out!
The main problem with the 1NT : 2C , 2D : 2M sequence is the ambiguity in responder's major-suit length. It could be either 4 or 5 cards - and having to cater for both is a little uncomfortable. In particular, it makes a difference to the the type of hand on which opener wants to accept the invitation. When responder has only 4 cards in the major he is mainly interested in whether opener is minimum or maximum in high cards. But when responder has a 5-card suit, fit tends to be more important. In an attempt to cater for both types, Keri uses a 3C bid by opener to show a minimum with good fit - but of course when you do this you are losing the chance to play in 2M, which was supposed to be the advantage of playing Keri. And in any case this still doesn't solve the problem that there can be a big difference between 2- and 3-card support when responder has 5-cards, but there isn't when responder has only 4.
Another issue is that you frequently play in a 4-3 fit. Opener passes the 2M bid whenever he is minimum with 3-card support. As Klinger's book points out, this often works quite well. But it does rather depend on the hand. Playing a 4-3 fit looks great when responder has a good suit and opener has a weak side-suit doubleton. It looks rather less clever when responder's suit is bad and/or opener is flat. The difference is highlighted at matchpoints where you are going very much against the field (and if 2M makes the same number of tricks as no-trumps you will score badly). Unfortunately the system forces you to play 2M every time. So you end up with some good results and some bad results. Klinger's book seems to be trying to persuade us that it is a winner on average; this wasn't my impression from playing it.
There is no doubt that when responder has a five-card suit and the invitation is rejected, you are pleased to be able to play in 2M. But here Stayman can be even better than Keri, at least when the suit is spades. Playing Stayman we can use 1NT : 2C , 2red : 2S to show an invitational hand with 5 spades. When this comes up we get all the benefits of Keri but without the ambiguity about spade length. As mentioned above, this helps opener in knowing when to accept the invitation. But, even better, it means that opener can pass with a doubleton. A 5-2 fit does tend to play better than no-trumps, particularly if responder is unbalanced. In Keri you have to bid 2NT over 2S with a doubleton spade, in case responder has only four. But when responder has a 5-card suit you would generally prefer to play 2S in the 5-2 fit rather than 2NT.
This is not available when the suit is hearts. If we play Stayman we have to start with a transfer on an invitational hand with 5 hearts. My preference is that opener should be keen to super-accept with a good heart fit, so when responder has a borderline invitation he should transfer and then pass 2H. This can also work better than Keri on those hands (since we play 2H rather than 2NT opposite a doubleton). On the other hand, a sound, fairly balanced invite with hearts is certainly better bid in Keri.
One nice thing about Keri is the continuations after a transfer: because 2NT is not needed as invitational, it can be used to improve the game-forcing sequences instead. However, this is not really an advantage of Keri since it can also be easily incorporated into a Stayman-based system. Playing Stayman-then-2S as an invite frees up 1NT : 2H , 2S : 2NT just like in Keri; and if we are playing Stayman we don't need to use 1NT : 2D , 2H : 2S for 4-4 majors like Keri does, so there is plenty of room here as well.
There are more problems if you play the "book" version of Keri. One that particularly bothers me is that while Stayman is notorious for giving away information to the opponents unnecessarily (when opener shows or denies a major that responder is not interested in), Keri is, if anything, even worse in this respect. After 1NT : 2C , 2D : 2NT opener reveals whether or not he has a doubleton, even though this information may not be needed by responder. A similar thing happens after 1NT : 2C , 2D : 2M if opener has enough to accept the invite. Against opponents who are good enough to make use of the information this is a very bad idea.
Finally, you do need to think carefully about how you will cope with interference, and the book is a little short on detail here. I remember once, after bidding 2C and hearing a 2S overcall, not being sure whether 3D would now be weak or invitational. (It may not even be possible to distinguish.) In the Junior Camrose this year a player responded 2D to 1NT on an invitational hand with 5 spades and 4 hearts, and then heard a 2S overcall! This should have gone for a four-digit penalty, but instead they had a misunderstanding and ended up in a silly contract. A transfer which only guarantees 4 cards does tend to cause problems in competition even if you are well-prepared. More generally, you'd better be sure you know how much of the artificiality applies in a competitive auction. This significantly increases the amount of work involved.
My conclusion is that while Keri is a decent alternative to Stayman, it certainly isn't "Much Better" as the title of the book claims. Perhaps the best thing about the system is that if you and your partner both know the book, you can agree "Keri" and immediately have a complex, rather effective system ready to go, without needing to develop it for yourself. But, in my opinion, if you had a book which was based on Stayman that could be even more effective.