With an agreed suit and a strong hand it is useful to know the number of aces in your partner's hand to evaluate whether to bid a slam. Blackwood, Gerber and Roman Key Card are conventions designed so that you can count aces. But they do not show which ones.
Any new suit bid after the agreement of a suit is a cue bid. This shows control in that suit. When a cue bid is used it shows an interest in a slam.
First round control can be the Ace or a void.
The suit should be the lowest in which the bidder has control. This allows partner to bid a suit at the next lowest level in which she has control i.e. bidding ‘up the line’.
1S - 3S - 4C (control in clubs) - 4D (control in diamonds)
Missing a suit up the line denies control in the missing suit.
1S - 3S - 4D (shows control in diamonds but not in clubs)
The repeat of a suit in a second round of cue bidding shows second round control in that suit.
1S - 3S - 4C - 4H (shows control of hearts but not diamonds) - 5C (shows second round control of clubs)
Where a suit is bid before a suit is agreed that suit becomes a cue bid after the suit is agreed.
1D - 1S - 3S - 4C - 4D - (4D now a cue bid)
With this information a decision on the contract can be made without undue risk.
Cue bidding should not be used over an opening bid of 1NT or 2NT where Gerber or Blackwood is preferred.
A double jump response in a new suit is called a splinter. 1H - PASS - 4C
A splinter bid shows four card support in Opener’s suit and first round control in the new suit ie. an Ace or a void.
The responders hand should have at least 10 points since a game level contract is the minimum rebid.
Any bid of a new suit after the splinter is a cue bid showing an interest in a slam.
Michael’ is an Overcall that is used to show a two suited hand.
Unusual NT is a Cue Bid that is similar to Michael’s. It is an Overcall that shows the two lowest unbid suits.
If you use an UCB after Partner’s Overcall, you are promising at least 3 cards in Partner’s suit and 10+ points.
The bid is not game forcing so the Opener can rebid her opening suit depending the strength.
eg. 1C - 1S - PASS - 2C (“I have at least 3 Spades and 10+ points”).
Remember that a game in a Minor requires 27+ points, therefore the UCB work better in major suits.