You should be acquainted with the articles on 2-way checkback Stayman to fully profit from this article.
The XYZ convention is basically the extension of the artificial bids discussed earlier to any auction beginning with 1X-1Y-1Z, hence its name. In this framework, 2 Clubs will always be ART, forcing partner to bid 2 Diamonds, either to play in that contract or to begin an invitational sequence; 2 Diamonds will always be ART, forcing to game; 2NT will always be ART, forcing partner to bid 3 Clubs, either to play in that contract or to show a game-forcing two-suiter.
There are some important differences between auctions involving a NT rebid and auctions involving a suit rebid, though, and they must be taken into account by partnerships using the XYZ convention. A suit rebid is much less well-defined in strength and in distribution. 1 Club - 1 Heart - 1 Spade, in many partnerships, can show a 4=3=3=3 or a 5=1=1=6 hand or anything in between, and the strength can range from 12 to a bad 18 (since a jump to 2 Spades, in a standard system, would force to game).
What this all means is that, unlike auctions in which opener's rebid was 1NT, opener may still have plenty of surprises that require special handling.
That said, for the sake of simplicity and because the extreme cases are very low frequency, I recommend that the meanings of the bids by responder remain the same, and that opener should make an effort to make the expected bid. But this is not an absolute rule. If you have a 65 and the bidding was 1Club - 1 Heart - 1 Spade - 2 Clubs, your proper bid is surely 2 Spades (and not 2 Diamonds), destroying partner's beautifully planned sequence. Showing 5 spades is a higher priority here. Other exceptions (in distribution and in strength) should be dealt in the same way, with opener making his most natural (and unexpected) rebid if he has an unexpected hand.
Let us examine a typical sequence and the most common follow-ups.
1 Diamond - 1 Heart
1 Spade - ?
Responder's rebids and their meaning:
Pass - Yes, this is a non-forcing auction, and pass is a possibility. Any hand with 3 spades, shaky stoppers, and weak (say, 5-7) should pass. For example, Kxx Q10xxx xx Qxx is a clear pass of 1 Spade.
1NT - Weak, suggesting a contract. It does not promise a stopper in the 4th suit; it merely denies a fit and strength to bid higher. xx Q10xxx Kx Jxxx is a 1NT bid. I'm giving examples with 5 hearts to emphasize that you should not rebid hearts merely because you have 5 of them, but of course there are many other examples with only 4 hearts.
2 Clubs - Forces 2 Diamonds, to play there or to begin an invitational sequence. Exactly the same as in 2-way checkback Stayman.
2 Diamonds - Artificial, game-forcing. This specific sequence is the most dangerous "memory trap" lurking for those who are still getting used to the convention. After decades of bridge, it is easy to bid 2 Diamonds with a hand such as xx Q10xxx Kxxx Qx, only to remember, too late, that this is an artificial, game-forcing bid. If you want to play in 2 Diamonds, you must begin with 2 Clubs. With that caveat, the bid is exactly the same as in 2-way checkback Stayman
2 Hearts - weak, rebiddable hearts. Note that, unlike a 1NT rebid by opener, it is very common for opener to be short in hearts in this auction, and so you should rebid hearts only with 6 of them. Qx K108xxx Qx xxx would be an ordinary 2 Hearts bid.
2 Spades - weak, 4-card spade support. I recommend that any hand with 4 spades should raise partner, unless you are really so weak that you perhaps should not have responded 1 Heart to begin with. Fits should be announced. That said, this is not a forward-going bid. Any invitation begins with 2 Clubs.
2NT - Forces 3 Clubs. As in 2-way checkback Stayman.
3 Clubs - weak distributional invite, typically 4-6. xx Kxxx x KQ10xxx is an example.
3 Diamonds - weak distributional invite. Being a raise, it does not require 6 cards. x Axxx KJ10xx xxx.
3 Hearts - to maintain continuity with what was recommended in prior articles, this cannot be invitational (it would begin with 2 Clubs), and is therefore a choice of games with a good 6-card heart suit. It is inviting partner's opinion between 4 Hearts and 3NT. Ax KQJxxx x K109x
3 Spades - Not invitational. It is not sensible to use a bid in a known 4-4 major suit fit to offer a choice of games, and so I recommend it to be a GF raise with an unknown void. 3NT can ask for the void. Only use this bid if you and your partner are confident of not forgetting it.
3NT - to play with a balanced hand and club stoppers
4 Clubs / 4 Diamonds - spade raises with a singleton in the bid suit (yes, even in partner's suit).
4 Hearts / 4 Spades - to play. 4 Spades, particularly, should be very unsuitable for slam, since it leaves so little space for partner (who may be quite strong).
You may deduce similar lists of meanings for the analogous auction 1 Club - 1 Heart - 1 Spade.
Next article will deal with 1 Club - 1 Diamond - 1 Heart, and 1 Club - 1 Diamond - 1 Spade. These auctions require a different approach because, in the Walsh style that we recommend (bypassing diamonds to show a 4-card major if you don't have strength to force to game), 1 Diamond denies a 4-card major unless you have 12+, and this changes the meanings of the bids.