Control bidding is done with strong hands to examine the feasibility of a slam contract.

When it is your turn, while bidding controls, you call the cheapest suit in which you possess a not yet mentioned control. Aces and Kings are always controls. When a trump suit has been established voids and singletons are also controls

The exchanging of controls is started when:

• Jacoby 2NT response is given to a 1-level major suit opening.
• After a suit has been established as trump suit and a different suit is called. This different suit contains not necessarily a control if it was the cheapest call.
• A transfer bid is refused. (the calling of NT after a transfer is not a refuse)
• After a cue bid (a call in the opponent's suit). An exception is made for the Ghestem-cue after an opponent's opening bid, which has a completely divergent meaning.
• After a strong minor suit opening via the 2¨ bid, and a second response, without jumping, in the other minor suit.
• After a strong minor suit opening via the 2§ bid , and a second response, without jumping of 3/4§.
• After a game bid, if one of the players calls another suit, unless it's clear that this move has been done out of fear.
• After a 3¨ answer on a 1NT opening or a 4¨ answer on a 2NT opening, executed via a 2-level opening bid in one of the minor suits.

The control bidding or exchange of controls is at least continued until the game level is reached.
Normally the initiator (the player, who gives the initial sign and who often has hidden strength and/or most information) of the control bidding decides about:

• The final level of bidding; if he bids the game in the fitting suit his partner should pass. When the bidding proceeds beyond the game level, the initiator's partner has to pass too, when the initiator calls the Suit which has to become trump suit.
• Weather or not Blackwood 4NT will be applied.
• Weather the exchanging of controls should be continued beyond 4NT, then 5NT is an Aces asking bid.
• The nature of the final contract: NT or the Suit. If 4NT has been called, then 5NT is eventually a final bid

If you want know more about slam bidding (also for grand slam) you should read the Blackwood page, where a suggestion is done about additional agreements that can be in this respect.
In that chapter you may also find a complete list of situations, where a trumpsuit is estabished.

When you want to see bridge games, in which the principles, described above, are applied, you should click for examples, and make your choice when you're arrived there.