The question is valid, why does the defensive side need a different system from the offensive side. It is a very simple reason: when the opponents begin the bidding at the one-level, they claim to have a least 12 of the 40 high card points of the deck. So we have to create a defensive system, which contains this fact.
When the opponents make a pre-emptive bid, they have taken a lot of bidding space from us, which was, in part their aim.
These two reasons are in themselves determinant factors for the need of a different bidding system to situations where we open the bidding.
The aims of interventions: The simple overcalls (e.g. 1♣-1♥ or 2♥-3♦) have three, partially distinct aims:
- Construction, that is, the maximal utilization of the total strength of our line, the investigation of a game. Following a one-level opening, we are unlikely to have a slam, as the opponents have taken 13 points of the 40, unless we are short in their strong suit.
- Competition and interference. It goes without saying that by our bids we can force the opponents to make risks and we can also take bidding space from them, making accurate communications difficult. This goal has been in the foreground in the strategy of modern competitive bridge. Like other sports, bridge has become more aggressive, the opponents try to occupy the so called key positions to make life more difficult to each other.
- To influence the opening lead and the defence. A suit overcall is often important because it pinpoints to partner which suit should he attack with. The bidding of a good suit is especially important in case of a no trump contract. If we show a good suit to our partner, then most of the time it is right to choose that suit for the opening lead and it is nearly always obvious from our hand when it is not (when we have an even better alternative).
It can be seen that the second aim (interference) is in contrast with the others. For this reason we tend to give a constructive definition when defining the overcalling hand, but to show some leniency towards interference, we can overcall on hands weaker than opening hands.
What options do we have, when we want to overcall?:
1.Simple suit overcall
Good five card suit hand and near opening strength hand (min 10 (9) HCP. In reopening position it may be 2 points weaker.
2. Double: take-out double
Either shortness in opponent’ suit and 4-cards in the remaining majors with normal overcall strength, or the initial bid of strong hand types.
3. New suit jump bids: preempts, as if we have opened
4. 1NT overcall: 15-18 HCP, balanced, stopper in opponent’s suit
Important warning: If we have a strong hand, but we only have length in the opponent’s suit, we have to pass! (See trap pass.)