There are many defenses against an opposing 1NT opening. Some are quite complicated, but here we will look at one of the simplest and most widely-played, the Landy Defense:
2♣ shows both majors, at least 5-4 shape
2♦, 2♥, 2♠ are natural
Double is for penalties
The defense operates against the weak or the strong 1NT. It's true that a penalty double will not be productive so often against a 15-17 1NT, but at least partner will be able to bid a long suit when he knows that there is a healthy point-count opposite.
See how it works in this quiz, where the opposing 1NT will always be 15-17.