In contract bridge, a cue bid (also, cuebid or cue-bid) is a term that applies to two types of bid:
After the opponents have bid a suit, a cue bid of that suit is normally intended as a forcing bid. It shows interest in contesting the contract and asks partner to describe his hand.
An immediate cue bid is made directly over opponent's opening bid. Traditionally, it denotes a hand unsuited for a takeout double. For example, after RHO opens 1♠, a hand such as ♠ - ♥ AKQ10985 ♦ AQ6 ♣ K85 would prefer not to double for takeout, because partner might make a penalty pass. A cue bid of 2♠, as traditionally used, would be appropriate: it tends to show great high card strength, probably with a hand pattern unsuited to defense. Partner is expected to respond in his longest suit, and the subsequent bidding proceeds naturally.
However, those very strong hands are rare enough that the traditional meaning has been largely abandoned, and other meanings assigned to the immediate cue bid. The most common treatment is now the Michaels cuebid, which shows a weakish or moderate hand with at least 5-5 in two unbid suits.
The immediate jump cue bid of opener's suit has a specific meaning. It is typically a long totally solid minor with stoppers in the other two suits. Partner is asked to bid 3NT with a stop in the suit opened or else to bid four or five clubs (pass or correct).
Generally, after the opponents have bid a suit, a cue bid of that suit shows strength, and forces the bidding to continue for at least one round. The following are common situations:
Once a trump suit has been agreed and the bidding cannot die below the game level (e.g. 1♥-3♥, or ... 2♥-3♥, or 1♥-1♠; 3♠), any subsequent bid of a suit other than the trump suit is a cue bid showing first round control of that suit, i.e. the ace or a void.
Passing a suit that could be bid tends to deny holding first-round control in that suit. Bids of suits already bid show second-round control. Returning to the trump suit shows a lack of interest in slam or not having anything else to bid. For example:
South has shown 16-18 total points, while North's hand is largely unknown. North's bid of 4♣ is a cue-bid showing first-round control of clubs and an interest in slam. After South's bid of 4♦ North bids 4♥, an apparent signoff. It may well be that North wants to bid a slam, but has two fast losers in the spade suit. After South bids 4♠ showing control of the spade suit, North employs the Blackwood convention to proceed further.
The main disadvantage of both Blackwood and Gerber is that they give little information about voids, which can be as powerful as aces under certain circumstances. Cue bidding is designed to pass information on "first round control" i.e. an ace or a void.
In the "Italian" system of slam cue-bidding, the cheapest suit is always bid first. Thus, in the example above North's bid of 4♣ would deny control of spades, and therefore South would only proceed if he had control of spades, which in this case his continuation of 4♦ instead of a signoff of 4♥ would promise. Often, Italian cue bids only promise 2nd round control (a king or a singleton).
Basic cue bidding occurs after the trump suit has been agreed explicitly (example above) or as it is agreed implicitly. The first bid of a side suit by either partner shows a control, and most players extend that to subsequent bids of a side suit so that both may show control in the same suit. The most common approach is that first-round controls are bid first, and second-round controls are bid in later rounds of bidding. Some players, though, bid both first and second-round controls in the first round, and confirm first-round controls only later. Some bid high-card controls first or distinguish short-suit controls by jump bids. Even basic cue bidding therefore requires some partnership agreements.
The Official Encyclopedia of Bridge gives an example where the first cue bid implies the trump-suit agreement.
|♠||A J 8||
|♠||K Q 10 6 3|
|♥||K 10 7 6||♥||A 3|
|♦||A Q 10 7||♦||8 3 2|
|♣||K 9||♣||A 8 6|
"West's 4♦ does not suggest an alternative trump suit; spades are agreed by implication, since without spade support, West would return to 3NT. "
An advance cue bid - not to be confused with "advanced" - is made before the trump suit is agreed even implicitly. Partner does not yet know that the trump fit has been found or whether the suit bid is real or shows a control.
|♠||K Q 8||
|♥||A J 9 6 4 2||♥||K Q 3|
|♦||K 3||♦||A Q J 7 5 2|
|♣||10 3||♣||A 8|
In the example, 3♣ is an advance cue bid, we may infer East intends, by reference to the club control without biddable club suit and to the excellent heart support. The subsequent preference for hearts by strong correction of 3NT should reveal the advance cue to partner; holding the minor suits without hearts East would pass or raise notrump. The auction should now proceed to the comfortable 6♥.
However, the advance cue bid is a subtle tool, prone to be misunderstood if the partnership is not sufficiently well coordinated and on the same wavelength. The following disaster struck world champion Paul Chemla and Catherine D'Ovidio in the 7th European Mixed Championships (2002):
|♠||K J 10 9 8||
|♠||7 4 3|
|♥||A Q 3||♥||K J 9 8|
|♦||A 9||♦||K 3|
|♣||A K 3||♣||Q J 7 5|
1. Strong and artificial.
3. Ambiguous but forcing.
D'Ovidio wanted to "refine" the bidding with an advance cue bid of 4♦ before supporting spades, but Chemla got the wrong idea and passed, imagining partner with a weak distributive hand with long diamonds. 4♦ went down three, while 6♠ and 6NT were on, as the spade finesse was working.