XYZ is a convention designed to allow partnerships to find suit fits at a low level. It is also handy in 2/1 when bidding starts 1 - 1 - 1 or 1 - 1 - 1N where responder could have any number of HCP.

"XYZ" stands for 3 bids made at the 1 level, such as 1-1-1 or 1-1-1N.
There are 10 different combinations of such 1-level bids.

There are also several combinations of such bids when opponents intervene, such as
1-(1)-D-(P)-1N where the partners only bid 2 actual suits between them.

If opener makes his second bid on the 1 level, XYZ is on.

Responder can then make one of the following bids: 

    Weak sign-offs: 
      2/3 = sign-off. This would logically show a very weak hand with 6+ of the suit bid.
      To sign off in 2D, first bid 2 (relay to 2) then pass. 
      2N = balanced, 10-11 HCP.
      2 = relay to 2 then bid 2//N as invitational. 
    Game Force
      3// = game force with 6+ of the suit bid.
      2 = any other game force.

To make an invitational bid, responder bids 2 first and then any bid after opener's 2 is invitational.

    For example, in the Feb.2017 Bridge Bulletin, p.38, The Bidding Box:
      86 AK74 972 KQJ5
      T7 832 AKQJ8 T72

      The bidding went

        1 - 1
        1 - 2 (XYZ)
        2 - 3 (invitational)
        Pass (for a top)

    In the March 2017 Bridge Bulletin, p.42, #3, with Q32 AKQ64 T43 42 and bidding of

      1 - 1
      1 - ??

      Bids by the panel were all over the place and the reasons for the bids were painful.

      • 2 - "overreaching a bit"
      • 2 -
        • "I choose 2 because the coin came up tails."
        • "Notrump looks wrong and hearts could be a poor fit."
        • "[2] - to keep the bidding open"
        • "I don't like raising spades with only 3."
      • 1N - "Very heavy for this bid, but there is no good alternative."
      • 2 - "Bidding NT is unattractive with such poor diamonds..."

      In XYZ, a bid of 2 at this point transferring opener to 2 makes the next bid of 2 invitational and is clarified better than by bidding it without XYZ.

    In the Dec. 2020 Bridge Bulletin, p.40, with KQ2 KJ74 Q75 953 and bidding of

      1 - 1
      1 - ??

      The caption for the quiiz answers was Little Lies with 5 experts bidding a conservative 1N, 4 bidding 3, 3 bidding 2 and others bidding 2, 2 or 2.

      One expert (Molson) said: 

        Lucky XYZ bidders. This is an impossible hand and probably why the convention was developed. There is no good bid [without XYZ] Values for an invitation but no stopper in clubs or four of either of partner's suits.

      Mike Lawrence, playing XYZ, bids 2, relaying partner to 2, then says:

        I will follow with 2. This odd sequence ought to show invitational values with three spades.

      Note that with the bidding shown above, 2 is not a weak suit preference bid in XYZ but is actually a game force. A weak suit preference would be to relay to 2 then pass. An invitational diamond raise would be a 2 relay to 2 and then bid 3.

      If diamonds are your suit (e.g.: 1-1, 1any-??), a bid of 3 here is game forcing and shows 6+ diamonds.

    Larry Cohen adds the following XYZ tip on his web site: 

      There are several ways to invite (via 2 first, or sometimes directly). Serious partnerships can invent their own meanings for the difference. For example, define a different meaning for 1-1-1NT-2NT and 1-1-1NT-2-2-2NT. Perhaps going through the relay of 2-2 is a 5-3-3-2 invitation with 5, whereas the direct 2NT denies 5.