Source: Wikipedia

Negative free bid is a contract bridge treatment whereby a free bid by responder over an opponent’s overcall shows a long suit in a weak hand and is not forcing. This is in contrast with standard treatment, where a free bid can show unlimited values and is unconditionally forcing. The treatment is a relatively recent invention, and has become quite popular, especially in expert circles.

Negative free bids resolve relatively frequent situations where the responder holds a long suit with which he would like to compete for a partscore, but is deprived from bidding it by opponent’s overcall.

Example:

West	North	East	South
	1	1	?

For example, if South holds: 86KJ10852 K6 532, partner opens 1and East overcalls 1, he couldn’t bid 2in standard methods, as it would show 10+ high-card points, and a negative double would be too off-shape. With NFB treatment in effect though, he can bid 2which the partner may pass (unless he has extra values and support, or an excellent suit of its own without tolerance for hearts).

However, as a corollary, negative free bids affect the scope of negative double; if the hand is suitable for “standard” forcing free bid (10-11+ points), a negative double has to be made first and the suit bid only in the next round. Thus, the negative double can be made with the following types of hand:

• A weakish hand with unbid suits (unbid major)
• A stronger hand with unbid suits
• A strong (opening bid or more) one-suited hand.

This can sometimes allow the opponents to preempt effectively.

West	North	East	South
1	1	Dbl	4
?

For example, West, holding: KJ103 J8 AKQ104  J2, after this auction is in an awkward situation — he doesn’t know whether partner has spades or not; whether South was bidding to make or to sacrifice — is it correct to double, bid 4or pass?