After a trump suit has been agreed, most side-suit bids above 3 of the agreed suit are control bids. A control bid shows both slam interest and a first or second round control in the suit bid.
- Partner is saying that they either know the partnership has the 33+ total points needed for slam, or is at least close. Aggressive partnerships can use control bidding when the partnership is known to have 29+ total points. More conservative partnerships may prefer 31+.
- A first-round control is either an ace or a void (i.e., you can gain control the first time that suit is led).
- A second-round control is either a king or a singleton (i.e., you can gain control the second time that suit is led). Only treat a king as a second-round control if it isn't likely to be led through (or you know that you or your partner has the ace).
In order to make a slam, the partnership must have at least second-round control in all three side suits. Therefore, Blackwood should not be used by a player that has a side-suit holding like Qxx until they have ascertained that their partner holds first or second round control in that suit. A player who has a void should also control-bid their void instead of going directly to Blackwood, although this is not without issues either.
When a side-suit bid above 3 of the agreed suit is not a control bid
- Control bidding can only be initiated after partner has already made a bid that unambiguously communicates suit agreement. This contrasts with Blackwood, which can sometimes signal agreement by itself.
- In a sequence such as 1♥ - 2♥ - 3♠, the side-suit bid is a splinter, not a control bid.
- To control bid after partner has raised to 2M, start by bidding a side suit without jumping, as an artificial relay; then control bid over partner's response.
- You can respond to a splinter with a control bid.
- Jacoby 2NT has a set of conventional responses. See here.
If partner has initiated control bidding, in order to continue, make sure you have the undisclosed extras needed. For example, if you and your partnership have agreed that initiating control bidding requires the partnership to have 31+ total points, and your partner has initiated control bidding, you need at least 2 more points than you previously showed in order to be sure that you have 33+ total points together. If you do, then you can continue bidding controls. If not, you need to sign off in the agreed trump suit at the cheapest level. If partner is strong enough to still be interested in slam opposite a minimum, they can continue bidding controls anyway.
If partner skipped over a suit, thereby denying controls in those suits, first make sure you have at least second-round control over the skipped suit. For example, in the sequence 1♥ - 3♥ - 4♣, opener denies second-round control in spades. If you also don't have second-round control in spades, it means the opponents can take two spade tricks off the top. Put the brakes on by signing off in 4♥. Likewise, if partner skipped over two suits, as in 1♥ - 3♥ - 4♦, make sure you have at least second-round control in both suits, and first-round control in at least one of them. If not, it means opponents have two quick tricks (one in each suit) so you need to put the brakes on.
Note that if you continue showing controls after partner has skipped a suit, it is understood that you are implicitly showing controls in the skipped suits.
Bidding a suit that you've already shown a control in tells partner that you have first-round control (as opposed to merely first or second round control).
Once you're satisfied that the partnership has second-round control in all suits and enough strength for slam, bid 4NT to ask partner about first-round controls and trump keycards. You should do this as soon as possible, otherwise the control bidding may go above 4NT. If you've already bypassed 4NT, you'll have to continue bidding controls and use your judgement.
If you have no more controls to show, return to the agreed trump suit.