Forcing notrump

The forcing notrump is a bidding convention in the card game of bridge.

In Standard American bidding, the response of 1NT to an opening bid of 1♥ or 1♠ shows 6 to 9 high card points (HCP) and is non-forcing. Opener, with a balanced minimum, may pass the 1NT response and, if the opponents also pass, that will become the contract.

A partnership may agree that this bid is forcing for one round; if the intervening opponent passes, opener must bid at least once more. This guarantees the responder at least one more opportunity to bid or pass. This mechanism allows the partnership to use the 1NT response for a greater variety of hands: in particular, invitational as well as minimum responder holdings. The forcing notrump is used over major suits only; 1NT is always standard and non-forcing over the minor suits.

The forcing 1NT bid shows 6 to 12 HCP, denies the ability to make a single raise (but not necessarily an invitational raise), and denies holding four spades if the opening bid was 1♥; it must be announced as "forcing" by partner.

As the forcing notrump creates problems of its own, a popular variation that overcomes these is the forcing next step.

Opener is forced to bid again:

When opener does not have a six-card original suit nor a four-card lower-ranking second suit, she will have to bid a short suit. Normally, she bids her three-card minor. If she has three cards in both minors, she bids 2♣.

If opener holds exactly four spades, five hearts, two diamonds and two clubs (and thus originally opened 1♥), she bids 2♣, a two-card suit. There is no point in showing the spades, because responder has denied holding four spades (having skipped the 1♠ response to bid 1NT). After a 1♠ opening, however, there is no distribution that would compel opener to rebid a two-card suit.

...
Wikipedia