Source: Wikipedia

After opener’s one of a suit (X), partner’s one of a suit response (Y) and opener’s 1-level rebid (Z) (a very common sequence), 2from opener’s partner forces 2from opener. Holding a weak hand with diamonds, one can now pass. Other bids are invitational, describing also the nature of the hand. A 2rebid by responder, however, is game forcing with any hand. Responder may also jump to 3 of any suit (except 3, which is signoff) to show a game-forcing hand with a good suit or, if the jump is in one of opener’s suits, two of the top three honors.

Although it is mandatory in the XY Notrump form of the convention to complete the relay 2to 2, when Z was a suit, opener may be unlimited. In such a case it is acceptable, if opener has too good a hand to risk being dropped in 2to bypass the relay and make a (forcing) bid.

It is also customary to retain the convention in the face of certain competitive actions, for example after a double by opener’s LHO, or a negative double by partner. After an overcall by RHO, assuming that the bidding has not gone past 1NT, the convention is still on (for example after 1– (pass) – 1– (1) – X where X is a Support Double).

Examples:

For example, on the sequence 1-1(with or without opponent’s interfering);1NT-2;2,2 will show an invitational hand with minimum five spades and four hearts, 2an invitational hand with minimum five spades (in which case one does not have four hearts) or possibly six spades with or without four hearts. 2NT will show a balanced invitational hand, 3typically an unbalanced invitational hand with club support. However, when one instead forces game with 2, opener will immediately start describing his own hand. He may, on basis of the aforementioned auction, with 2 being the only difference, bid 2holding four hearts, 2holding three card spade support, etc. Further bids are primarily natural.

Advantages and disadvantages:

XYZ gives very good control finding both games and slams and, in the case of a minimum opening, can often result in playing at the two-level where other pairs are at the three-level. On occasion, you can also arrange to play in 2(by simply passing over the relay).

There is of course the disadvantage that the partnership can never play in a contract of 2. However, when that is a good contract, the opponents will seldom let you play there and when it’s a bad contract? Well, why would you want to play there?

Playing, as very common in combination with XY no trump, the Walsh convention or even Transfer Walsh, one still has the chance of stopping in 2on weak hands, often impossible using for example Checkback Stayman.

Variations:

Most of the variations arise in the sequences that follow the 2 – 2 relay. For example, the exact distinction between the following sequences:

   1 - 1 - 1 - 2 - 2 - 2NT

and

   1- 1 - 1 - 2NT

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