When your opponents have opened the bidding and you have a strong, balanced hand you can still bid 1NT BUT with one extra feature, which is a ‘stop’ in their suit.  That is, good enough cards in their suit to stop them taking a bunch of tricks in that suit.

An overcall of 1NT still shows at least 15 HCP, ideally 16+ and like all overcalls, an upper limit of 18 because with more than that you start with double. With exactly 18, balanced, and with stops in their suit, it’s a judgement call: You can bid 1NT straight away, and partner will know you have 15-18 or you can double first and then bid NT showing 18+.

You will see from the panel that after an opening bid and two passes you can overcall 1NT with a less strong hand. The reason for this is that the opener’s partner must have a very weak hand when they pass their partner’s opening bid, so your partner is very likely to have some points. This is called making a protective bid – you are helping out your partner in situations where they have values but no natural bid they can make.

One very important thing to remember is that for your partner, the responses to a notrump overcall are the same as for a NT opening, with Stayman and transfer bids.