For example, after RHO[1] 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 their longest suit, and the subsequent bidding proceeds naturally.
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.
North's bid of 4♣ is a cue-bid showing first-round control of clubs and an interest in slam.
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.
Partner does not yet know that the trump fit has been found or whether the suit bid is real or shows a control.
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): W E1.