Control Showing Cue-Bids

This convention is used to be assured that the opponents could not immediately win two hands in one suit. When you intend to play slam, you should search this after you agreed on a trump. After agreed on a trump, a shifting suit will be a control showing cuebid and invitation to slam. If partner agrees then continue to show his controls. Controlled suit means that the opponents can not immediately win two hands from that suit. So, if you control any suit, you ever have the Ace or King, or that suit is void or singleton. While you are showing your controls you should not skip any suit. Skipped suit means not to be controlled. If there is not any uncontrolled suit then you apply Blackwood convention in order to learn the number of Aces. Let's give an example:

Opener	Responder
1 Spade 4 Diamonds	3 Spades
		4 Diamonds is a control showing cuebid and invitation to slam. But since Club is skipped, it means that Club is not controlled by the Opener. If the responder controls the Clubs, continue to show controls, otherwise he bid 4 Spades.