2/1 game forcing

2/1 game forcing (Two-over-one game forcing) is a bidding system in modern contract bridge structured around the following responses to a one-level opening bid:
 * 1) a non-jump response in a new suit at the one-level is constructive and forcing for one round,
 * 2) a non-jump response in a new suit at the two-level is forcing to game, and
 * 3) a 1NT response to a major opening is forcing for one round and indicates insufficient values to immediately commit to game or bid a suit at the one-level.

The 2/1 game force does not apply to responses by a passed hand, or if there is an intervening by an opponent. Other responses are per Standard American methods in accordance with.

Game forcing auctions
The 2/1 auctions are: 1–2, 1–2, 1–2, 1–2, 1–2, and 1–2.

Variations
The following variations may be made by :
 * Some play that 1–2 is not game forcing
 * Some play that 2/1 is not absolutely game forcing and the pair can stop below game when responder rebids his suit. For example, 1–2; 2–3 may be treated as nonforcing.

1NT response to major suit opening
Because the two-level responses are stronger than in Standard American bidding, the response of 1NT is forcing for one round and is used (among other things) for weaker hands containing low-ranking suits. Since the 1NT response is forcing, hands with a three-card limit raise can start with 1NT and later jump-support partner. See Forcing notrump for additional details. Some pairs play a variant in which the 1NT response to 1 or 1 is semi-forcing.

Since opener has been forced to rebid on hands which he might otherwise have passed, he may have to rebid in a new suit with only three or even two cards.

Other features
Use of the 2/1 system usually implies (at least) the following additional agreements:
 * Five card majors, i.e. major suit openings require at least a five-card suit;
 * Either limit raises of major suit openings (i.e. the jump raise 1 – 3 or 1 – 3 shows a game-invitational hand with at least four-card support) or Bergen raises;
 * Inverted minor raises, in which a jump raise of a minor suit opening is a weak preemptive bid, while a single raise is strong and forcing for one round;
 * Jacoby 2NT, showing strong support with 4 or more cards;
 * Splinter bids;
 * New minor forcing;
 * Fourth suit forcing and artificial.

Examples
1 – 2 2 – 2 Forcing to game, with original spade support and good club suit. This is different from standard bidding, in which such a sequence would show about 10 points, and club suit could be semi-fake.

1 – 2 2 – 2NT. Forcing to game, with balanced hand and a good club suit.

1 – 2 2 – 3 Forcing, unless the partnership has agreed that this is an exception to the "2/1 rule."

1 – 2 Forcing for one round only (as in Standard American), except in the variant of 2/1 where this sequence is game forcing as well.

1 – 2 Forcing for one round; 10 points or more with at least four clubs.

1 – 3 Weak; 9 points or less—sometimes much less—with at least five clubs.

1 – 2 Weak; 6-9 points with at least 3 hearts (unless Bergen raises are in use, in which case it shows precisely 3 hearts)

1 – 1NT; 2 – 2 Shows a weak hand, 6-9 points, with precisely two spades. Some also use this for an extremely weak hand (0-5) with three spades.

1 – 1NT; 2 – Pass Shows a very weak hand, perhaps 5-7 points, with at least four clubs.

1 – 1NT; 2 – 2 Shows a weak hand, 5-9 points, with a long diamond suit.

1 – 1NT; 2 – 2 Shows a weak hand, 5-9 points, with a long heart suit.

1 – 1NT; 2 – 3 Shows a weak hand, 5-9 points, with a long club suit.

1 – 1NT; 2 – 2NT; Shows 10-11 points without support for spades.

1 – 1NT; 2 – 3 Shows 10-11 points with 3-card support for spades.

1 – 3 Shows 10-11 points with at least 4-card support for spades.

1 – 1NT; 2 – 3 Shows 10-11 points with a long heart suit.

1 – 2 This is a jump response, and there are different ways of handling it. In Standard American, such a "jump shift" shows a very strong hand and is unequivocally forcing. However, since such hands do not occur with great frequency, it is more common today to use such a bid to show a weak hand with a long suit, unsuitable for defense. Another possibility is to play it as a "fit-showing jump", showing 8-10 points, a decent heart suit, and good diamond support.