In many classes, I get asked, "Larry, what should we do when they use a Convention (such as CAPPELLETTI, or DONT against our Strong Notrump?) There is no fast answer. To cope with interference, you need to do a little studying and memorizing. (Otherwise, be prepared for some accidents). You and your partner have to come to some agreements.
It is impractical (unless you are a world-class bridge pro) to try to memorize different schemes versus different methods. For example, to play System-A against DONT, but System-B against CAPPELLETTI, and System-C against Brozel would be too much memory drain.
Here is a medium-complicated scheme to deal with any kind of interference--you can choose parts or all of it. (For a simpler and shorter treatment, click here ).
THEIR OVERCALL PROMISES BOTH MAJORS
First, let's get a specific situation out of the way : The opponents show MAJORS. Most conventions over 1NT have a way to show MAJORS. If the opponents do show majors (with some contraption that is below 2), I suggest:
2 = GF in
2 = GF in
2NT = lebensohl (see below) -- Relay to 3
3 of a minor = Natural, Invitational
3 of a MAJOR = GF, Shortness (0 or 1) in that MAJOR
3NT = To Play, Neither MAJOR stopped
Use the 2NT mechanism to show stoppers -- after the Relay to 3 : 3 MAJOR shows only that major stopped; 3NT shows both MAJORS stopped. Relaying to 2NT also lets you sign-off in 3-minor (either pass the relay with or sign-off in 3 with )
NOTE: Using this method, when they show MAJORS, you can show either minor (Weak, Invitational, or Forcing) and can show which MAJOR(s) are stopped, and also can show both minors and shortness in a major.
OK -- That alone should take some brain-power to memorize, but I like it because it is mostly natural/logical (especially if you are already familiar with the idea of using lebensohl--FADS -- more on this below).
Now, what if they interfere with a bid that does NOT specifically show BOTH MAJORS?
If they make an artificial DOUBLE, just ignore it--system 100% on (Stayman, Transfers, etc.). If they make a penalty DOUBLE, then you should not play any conventions--not even STAYMAN. Use all bids as natural (even 2 and 2\). You can decide if you want to do something fancy with REDOUBLE (or use PASS to force a REDOUBLE to show a 2-suited hand).
THEY OVERCALL 2
I don't care what it shows (unless it is BOTH MAJORS). It can be , a transfer, a 2-suiter, a 3-suiter, an either-or, a reverse upside down poisoned dragon, whatever. Just IGNORE it. This is where you might have heard the term "Mirror Double" -- or "Stolen Bid." You just double to say, "Partner, I would have bid 2, Stayman." Double means they "stole your bid." All other actions are as if they passed. Just ignore them. 2 is what it would have been without their interference. All bids (even 2NT) are treated as if the opponent Passed.
THEY OVERCALL 2 or HIGHER
So far we can deal with any bid that promises BOTH MAJORS, doubles and any kind of 2 overcall. That leaves overcalls of 2 or higher. Again, it is best not to start asking questions and getting involved with which convention they use. Treat them all the same (unless 2 is specifically BOTH MAJORS). No matter what they bid, no matter what it means (other than MAJORS), here is the schedule:
BIDDING A SUIT (on the 2-level) = NATURAL, NF
BIDDING A SUIT (on the 3-level) = see below (lebensohl explained)
BIDDING 2NT or 3NT (see below -- lebensohl explained)
BIDDING A SUIT (on the 4-level) = all jumps to 4, are "Front of Card" -- Texas, Gerber -- whatever you use.
Exception: If you bid one of their KNOWN suits (a suit they have promised) on the 2-level, you might choose not to treat it as natural -- your choice.
USING LEBENSOHL and "transfer" LEBENSOHL
If you've read this far, you are probably familiar with at least some form of lebensohl. Click here to read more about this convention.
What about Doubling their Overcall?
I was afraid you'd ask. Again, I like to separate it into 2 classes, and not get overly complicated. First, let's say they promise BOTH MAJORS. I like to use DOUBLE as penalty-oriented. It says you can double at least one of their suits, and give partner a chance to double as well. It sets up a force. If you pass and then DOUBLE, that should be a lightish takeout. For example:
1NT (2=MAJORS) X = penalty oriented (says nothing about )Passing and then doubling their 2MAJ = takeout of that MAJOR
If their interference is anything else (on any level), use X as cards/negative. It doesn't "promise" the other major(s), but you try your best. Think of double as just saying, "partner, I have values to act, but no suit to bid and nothing else fits." For example, let's say partner opens 1NT and they bid2 to show and either MAJOR. I would DOUBLE with each of these hands:
A Q 5 2
10 9 7
J 6 5 3
A 5 2
A 9 7
6 5 4 3 2
J 5 2
K 10 9 7
Q 6 5 3 2
Again, pass and then double is just a lightish takeout, competing. I'd use these "negative doubles" over NATURAL overcalls as well as CONVENTIONAL overcalls. If you have a true penalty pass of their known suit, you can pass and hope partner reopens with a double.
Now you know why people like to use conventions to interfere with your notrump. If you aren't prepared, expect some major accidents. If you study this and adopt it, but your partner doesn't, you can also expect some big disasters, but at least you were warned. Expect some growing pains, some memory pains, but eventually you'll be better for studying this area of the game.