2/1 game forcing (Two-over-one game forcing) is a bidding system in modern contract bridge, where a non-jump two-level response to a one-level opening bid commits a partnership to at least the game level. It is based on Standard American bidding and has largely superseded it; the principal difference is that a full opening bid is required for a response at the two level to an opening bid of one of a major. Thus, the response of 1NT to 1♥ or 1♠ opening is forcing or semi-forcing.
Natural systems in general have the following features:
Level-1 suit bids denote at least 4 or 5 cards in a major suit, and 3 or 4 cards in a minor suit, with strength of about (11)12-20(22) high card points. The suit bid is generally the longest. The former criterion inflicts further classification into four-card major and five-card major systems.
Bid of 1NT always presents a balanced hand in a narrow high card points range. The common ranges are 15-17 or 16-18 HCP ("strong notrump") and 12-14 ("weak notrump").
Bid of 2♣ typically presents a very strong hand (23 HCP up).
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