You have seen hands which are worth a suit overcall because the suit was very strong but are not worth an opening bid. Now for the other side of the coin. Remember last time we considered three possibilities:
P1) Your partner has most of the remaining points. Therefore you should play in game.
P2) Opener’s partner has most of them. Therefore they will outbid you, probably to game.
P3) They are evenly split. It is probable that both sides can make a part score at the two or three level if their chosen suit is trumps, It is even possible, though less likely, that both sides can make game.
Consider these hands if RHO opens 1♥.
a) Pass. The hand is ideal for a INT opening bid but it would be futile to bid 1NT now. You are achieving nothing in cases P1) and P2). If P1) applies partner will bid at his turn. You will later see that different rules apply to him in fourth seat. If P2) applies you will be doubled and carnage will ensue. Neither have you any desire to compete in case P3) as your high cards are as likely to take tricks in defence as if you are playing the hand, There is no real discrepancy between your playing strength and your defensive strength.
b) Pass. You would have been delighted to open 1♥ but once you hear that hearts are their suit you have nothing to say.
c) INT. A 1NT overcall shows 16-18 HCP and at least one guard in the enemy suit. It is slightly dangerous (you may still be doubled) but you must say something in the interest of P1).
Overcalling against no trump opening bids
Your 1NT opening bid has two important merits. Firstly it uses up bidding space, making it harder for opponents to overcall. Secondly it gives precise information to your partner, so that if they do misjudge an overcall he is well placed to make a penalty double. If RHO opens 1NT and you have a good six card suit it is safe to overcall but balanced and two suited hands pose problems.
The most important difference against INT is that double is for penalties not take out. It is right to double a weak 1NT on almost any hand with 16 HCP. Consider these hands if RHO opens 1NT
a) Pass. Just because you would have opened 1NT doesn’t mean you should bid now.
b) Double. This is risky but correct. If opponents have the majority of HCP they could make INT doubled but if your side has the majority of HCP (more likely as you have more than the opener) you will pick up a healthy penalty. You have done your bit. it is now down to the two unknown hands, opener’s partner and your partner.
c) 2♥. A good suit and fewer than 16 HCP.
d) Double. Too strong for an overcall.
e) Pass. Nothing to bid for the moment. If you feel frustrated, thinking that partner almost certainly has a fit for one of your suits, you may well be right. As you gain experience you will learn that there are plenty of conventional bids available over a 1NT opening bid to show a two suited hand.
f) Double. You can double on fewer than 16 HCP if you have the necessary playing strength to beat 1NT.