Defense Over 1NT Openings

After your RHO opens 1NT, you have the following options:

        The Overcall.    A 2-level overcall promises a 6-card suit with at least 3 honors. This should be emphasized:

Most pairs play penalty doubles of overcalls, so overcalling with a 5-card suit is just too dangerous.
        You may also jump to the 3-level with a 7-card suit (or an excellent 6-card suit). You are almost sure to get doubled here, so be careful!

        Balancing Overcalls:    You can be a little more aggressive after the auction (1NT) — Pass — (Pass). It might be ok in this situation to overcall with only two honors, or even to overcall with a 5-card suit containing 3 honors.

        The Penalty Double.    A double of a 1NT opening is for penalty. Such a bid shows about 17 HCP (or 14 HCP and a good suit to lead). Partner should respond to the double as follows:

(1NT) — X — (Pass) — Pass:     Shows a balanced or semi-balnced hand. 2, 2, 2, 2:     Shows a weak distributional hand with a 5-card suit. Any Higher Bid:     Shows a good hand (with 8 or more points).

        Balancing Doubles:    A double in balancing seat shows only 14 HCP or so, but should be based on a reasonably balanced hand with some strength in every suit. You should have help for any suit partner wishes to run in defense, and your hand should have support for any suit in case partner wants to escape.

        Two-Suited Overcalls.    The best time to overcall a 1NT opening is with a two-suited hand (at least 5-4 in the two suits). With a two-suited hand, there is an excellent chance of having a fit with partner, but lousy prospects for defending against 1NT.
        In standard bidding, the only two-suited overcall available is the Unusual 2NT Overcall:

However, various other conventions have been devised to allow for two-suited overcalls after a 1NT opening: