NO TRUMP OVERCALLS:

By this point, overcalls should be fairly clear. Much of the same thinking that goes into suit overcalls also applies to NT overcalls.

Bidding 1NT Over One of a Suit:

This is perhaps the easiest of all overcalls to explain. If your opponents have opened a suit at the one level, and you have a balanced hand with 15-18 HCP and one good stopper (an ace or king in a 2-card or longer suit, a queen in a three-card or longer suit) in your opponent's suit, bid 1NT in direct seat. With 12-14 HCP, you may bid 1NT in balancing seat. The logic behind these point ranges should be apparent by now. The slight extension of the point range beyond those for a 1NT opening comes from your inability to bid a suit and jump bid in NT to show 18-19 HCP. The lower point range for balancing seat bids follows the same reasoning as for overcalling a suit in balancing seat.

Your partner may treat your balancing 1NT as if it were the opening bid. Thus, if your LHO passes, 2C is Stayman, and transfers are available. This makes a balancing 1NT overcall both powerful and risky. You can provide your partner with a great deal of information, but your weaker points means that you may get into weaker contracts that you might not like. Some partnerships choose not to alter the point range for a balancing seat 1NT overcall, but the better solution is to create a minimum HCP requirements for transfers accordingly (Stayman is still safe with 8+ HCP). Work this out with your partner.

Bidding NT Over Preempts:

When one of your opponents makes a preemptive bid, it's necessarily more difficult for you to bid NT. You now know that there is a dangerous 6-card or longer suit out there, and you know where it is. If that player gets the chance to establish that suit and run off a series of winners, your contract is almost certainly going down. To prevent this, you need at least one stopper, preferably two, in your opponent's suit. Interestingly, however, you don't need much more strength to make a 2NT overcall of a weak two bid. If you've got 15-20 HCP, you should be OK, provided you have his suit stopped.

A 3NT overcall is another matter. Here you are preempting a weak bid at the three level and you can count on opener's having a 7-card suit. Look carefully at your holding in that suit. If you don't have a second round stopper—an ace or king in the suit with at least one other card to back it up—don't bid 3NT. Even though he has a weak hand, responder, which is the stronger of the two, will almost certainly win a trick or two and use his short holding in his partner's long suit to get over there and set your contract. If you have the stopper, you can come in with 16-23 HCP. It's a gamble at the low end, but work out the placement of the points. Your 16 plus opener's 9 leaves 15 HCP between two hands. Your partner will average 7.5 HCP, so you will be close to 24 HCP as a partnership, and most or all of opener's points will be negated if he can't get into the lead in his long suit.

Bidding NT After Both Opponents Have Bid

DON'T DO IT! You would never come in with a NT bid after you have passed, nor should you bid a NT if your partner has passed since it would simply tell your opponents, who probably have the majority of the points between them, where all of the strength lies. If you were to be left in the contract, you would probably suffer a loss for more points than what opponents could gain by making their (probable part-score) contract.