The tables at right help to illustrate the limits of the scoring advantage to be gained in duplicate bridge by preempting or sacrificing when the opponents may be successful in making a game contract.
There are four possible scenarios: In summary, based upon the expectation that the opponents are likely to bid and make game, it is advantageous to preempt, compete or sacrifice for down three when vulnerability is favorable, down two when equal and down one when unfavorable.
A preemptive opening bid is one made on the second or higher level, typically showing a weak hand containing a long, strong suit.
An alternative approach is to bid the level suggested by the Law of Total Tricks, with the assumption that partner's hand has one third of the remaining trumps.
However, they are normally loosened in third seat, when the partner has already passed, so the opening bidder can be sure that the only side preempted are the opponents, and thus can bid with better or thinner values.
A partnership can preempt the opponents cooperatively, having discovered that they have an excellent suit fit but not much overall defensive strength.
For example, after the partner opens 1♦ and RHO doubles, the following hand is suitable for a bid of 5♦, outbidding opponents' major suit game in advance:
Bids dictated by the LAW are often sacrificial, but nonetheless produce consistently better results with proper play.
When viewed in context of the Law of Total Tricks, normal preemptive opening bids, described above, basically assume that the preemptive bidder's partner possesses two of the five to seven outstanding cards of the long suit—mathematically, the "expected" number based on equiprobable distribution of the missing cards.
The Law of Total tricks allows the opening bidder to raise such responses by the number of cards in excess of five in that suit.