Interfering with opponents' no trump

1NT is a very descriptive opening, and it helps a lot in reaching the right contract... or in blocking the opponents if the opener's partner doesn't have much in the way of points! Here are some conventions designed to make the no trump bidder wish he'd never opened no trump instead of vice versa. These conventions are based on showing single-suiter and two-suiter hands; with a balanced hand you would prefer to defend.

Landy:
Landy is characterized by a 2C overcall showing both majors, usually 5-5. Can be 5-4 depending upon points and vulnerability, so with equal preference partner should bid 2D. Other overcalls are simple and natural. Double shows a balanced hand similar to a 1NT opening yourself, and partner has the option to pass for penalty or take out if he is weak. 2NT is unusual, showing 5-5 in the minors.

Brozel:
Brozel dispenses with the penalty double, so it should only be used against strong no trump openings. Double shows a single-suited hand (six carder usually, but with extra points and good suit, a 5 carder ought to be ok) with opening values. Partner must bid 2C as a relay to partner's suit, but has the option of bidding his own six card suit, if any. If there is interference, double shows some values and says, "Go ahead, bid your suit."

for HSimple overcalls show two-suited hands with a heart pivot i.e. 2C for C & H, 2D for D & H, 2H and S, 2S for S and a minor, 2NT for both minors. As always, 5-5 and an opening hand is the basic criterion for interfering, modified by actual number of points and vulnerability. Jump overcalls are natural and preemptive.

DONT:
Disturb Opponent's No Trump is similar to Brozel but negates the chance of going to the 3 level unnecessarily. 2C shows C and a higher ranking suit, 2D shows D and a higher ranking suit, 2H shows the majors, 2S shows a weak six card spade suit. Doubling and then bidding spades shows an opening hand with six carder spade. If partner does not have suport for the lower ranking suit, he bids the next one saying, "Bid your other suit."

The obvious problem with DONT is that knowing a double fit would vastly change your bidding from knowing one suit and a possibly worse fit in the other. The flip side is that Brozel gives away a lot of information to the opponents. Another problem with DONT is that if you have a major and a minor, it is difficult to find a fit in the major. If you are playing MPs, this can be a real problem, though at IMPs I would say DONT is preferable to Brozel.

Cappelletti:
DONT and Brozel work well against strong no trump, but against weak no trump you must retain the penalty double. In Cappelletti, 2C shows a single-suiter hand (2D is the relay now), 2D shows both majors, 2H is H and a minor, 2S is S and a minor, 2NT is both minors. This has the additional advantage of knowing the major immediately, which is crucial when playing matchpoints. Many people play Cappelletti even against strong no trump because 1NT is difficult for the declarer to play if the dummy has no points and no entries, even if the declarer has 16 points.

TONT:
Transfer Over No Trump is an interesting convention based on the premise that the no trump bidder invariably gives away a trick when he's on lead. So, when overcalling in the direct position, transfers apply. There are several modifications of the above conventions to effect such a transfer according to who will become the declarer. For example, you could modify brozel such that 1NT-x-P-2C-P-2D is in fact a transfer to hearts, or 1NT-2C shows D and H, 1NT-2D shows H and S, etc. TONT is a useful if confusing convention to use. I've seen people playing it with Landy and 5-card suit transfers to minimize the confusion and maximize the chance of interfering, even if double-suited hands are more difficult to show.

The Precision 1C opening is another bid where it is important to interfere, or the opponents will reach their optimal contract. Two nice systems for this are Suction and Crash. Perhaps I'll explain them later, but you could do a google search if you are really interested.

Cheers,
Prashanth.