This is one of the most confusing and most-asked about "conventions."
It refers to when your side bids the opponents' suit.
Bidding the opponents' suit on the 3 level is obviously NOT natural. But, does it ask for a stopper (the Western cue-bid)? Ostensibly, the answer is "yes" -- but this is not etched in stone.
||West's 3 bid shows an invitational hand with 6+ clubs (and denies 3 since there was no support double). East's 3 bid is obviously not natural. Is it saying he has spades stopped? Is it asking his partner for a spade stopper? Is it a control-bid, showing the ace or king of spades? Is it a "Western cue-bid?"
Technically, the answer is "none of the above." But, for the time being, West assumes that East is looking for a spade stopper. If West has spades stopped, he will likely bid 3NT.
A typical hand for responder would be : 9 8 2
A J 9 3 2
K 5 3
-- Responder wants to bid 3NT, but wouldn't dare do so without spades stopped. He doesn't want to bypass 3NT, in case opener has, say,K 6
J 6 4
A K Q 8 7 5 3.
Over East's 3, West would bid 3NT with this hand.
If East happens to have spades stopped he would usually just bid 3NT himself. Would he need a diamond stopper, too? Not really. The key in such a situation is to have the opponent's suit (the one they are likely to lead) stopped and not worry about the other missing suit.
Might East have had something else in mind when he bid 3? Sure.
He might hold: A 9
A K Q 7 2
5 4 3
J 6 4
. In that case, he is control-bidding spades -- and will later raise clubs (he won't sit for 3NT--he has club slam interest).
Let's look at another auction where your side bids the opponents' suit on the 3-level.
||East's 3 bid is not natural. It is presumed to be a "Western Cuebid" -- looking for a diamond stopper. If opener has diamonds stopped, he should bid 3NT.
A typical hand for East would be : 7 4
A Q J 4 3
9 3 2
A J 2
But, as before, East could have other things in mind--maybe he is slamming with a diamond control. If so, he will make his intentions known later in the auction.
What if the opponents have bid 2 suits?
Now, things change. When there are 2 suits to cue-bid on the 3-level, you should bid the one you do have stopped. The cue-bid is "telling" -- not "asking." For example:
East can bid 3 with, say, : 6 5 4
A Q 10 8 7 2
or East can bid 3 with, say : K Q J
10 7 6
K Q J 10 9 7
Summary of "Western Cuebids" :
When cue-bidding the opponents’ suit below 3NT, think of it as “forcing.” Assuming the opponents have bid only one suit, It doesn’t tell nor ask (sounds like the former U.S. army policy). It doesn’t show nor deny. It can be made with many types of hands. The partner of the cue-bidder will typically bid notrump with a stopper. If the opponents have bid two suits, then the 3-level cue-bid shows that suit stopped or controlled.
note: A Western cue-bid is NOT a jump to the 3-level -- you must already be at the 3-level. Also, it doesn't apply when your side has found a major-suit fit to play in. If you open, they overcall, and partner bids their suit, that is, of course, a raise (limit or better--not a Western Cue). Example:
The 3 bid is not a Western Cue. It says nothing about diamonds and shows a limit raise or better (3+ trump) in spades.
Lastly, when asking or answering questions about such bids, don't even bother with the term "Western Cue" -- just explain what your partnership agreement is (either : "shows a stopper" or "forcing, not saying anything in particular.")
Normally, a "Western Cue Bid" is made on the 3-level. But, sometimes you can bid the opponent's suit on the 2-level as a general force. For a brief video panel discussion (Fred Hamilton, Larry, and Bernie Chazen) of a "2-level Western Cue" click here.