When a trump suit has been agreed, any suit bid above three of the agreed trump suit is a slam-try cue bid, even if it is a suit that has already been bid by the partnership.
A cue bid shows a control (ace, king, void or singleton) in the suit bid. If the cue bid is below the level of game in the agreed suit it does not promise extra values, although by inference it does suggest a suitable hand for a slam. But a cue bid above game certainly does show extra values, as well as the promised control.
A partnership should not head towards a slam unless there is an expectation of taking 12 tricks. But it is no use having 12 winners if the opponents can win two tricks first. Cue bids allow the partnership to check that no suit has two quick losers.
When a player makes a cue bid, not only does he show a control but he also shows doubt about whether a slam should be bid, or possibly uncertainty about whether a small slam or a gran slam should be bid. If he had no doubt as to what the contract should be, he would bid it straight away.
Therefore a cue bid by one's partner is a request either to pick the final contract or, if not certain oneself, to make a further cue bid as an aid to helping partner pick the contract. So a cue bid is a dual purpose bid - it both shows a control and requests help in finding missing controls.
There is no need for any agreement with your partner, since slam-try cue bids are considered natural and part of standard bidding.
But please note: you cannot play Gerber and slam-try cue bids, they are incompatible. Very few good partnerships use Gerber, because it gets in the way of exploratory bidding and it really isn't very useful anyway. Even over no trump opening bids Gerber is hardly worth its place in your system. My advice is not to use Gerber at all. It is not part of Standard English Modern Acol.
Author: Chris Burton
Gravesend Bridge Club