When the right-hand opponent opens 1♥ or 1♠, the immediate overcall of 2 NT shows at least 5-5 in the minor suits (that is, at least five clubs plus at least five diamonds) and, presumably, a weakish hand (6 to 11 points). If the next player passes, the partner is expected to select the minor suit that he/she prefers and bid it at the 3-level. The unusual notrump is one of the earliest conventions devised for the game. It extended the principle that when the natural meaning of a bid is not generally very useful, it is profitable to agree that it means something somewhat opposite. That is, while natural notrump bids show a strong hand with balanced distribution, the unusual notrump shows a weak hand with very unbalanced distribution.
The unusual notrump is used only after the opponents open the bidding. In addition to succinctly describing the hand to the partner, it also deprives the opponents of a lot of bidding space (that is, it has a preemptive value).
Many players extend the convention to have it show the "two lowest unbid" suits rather than strictly the minors. Thus, over 1♣, 2NT shows diamonds and hearts; and over 1♦, it shows clubs and hearts.
If the 2 notrump bidder bids again freely, then it shows a ''strong'' two suiter. To distinguish the weak and strong holdings, many partnerships agree ''not'' to use the unusual notrump for intermediate hands (about 12 to 14 points); they would simply overcall with one of their suits and show the other later if the bidding offers a chance. However, for two-suiters, many players apply losing trick count (LTC) instead of point count, as it more accurately depicts the offensive trick-taking potential of the hand. With LTC in effect, the required strength is about 4.5–7 losers, the latter only in favorable vulnerability.
Similarly, a 4NT overcall shows an extreme two suiter (usually at least 6-5, often 6-6 or 7-6) and enough trick-taking potential so that a 5-level contract can be made, or a sacrifice will likely be profitable. It shows the same two suits as 2NT would show. Note that 4NT is "unusual" only when the opponents open the bidding; otherwise, 4NT is normally played as the blackwood or a quantitative invitation to 6NT.
To counteract the unusual notrump, the opening side may employ unusual vs. unusual.
Recently, the Unusual 2NT has been used in opening seat showing a 5-5 minor suited hand and 7-11 HCP. The preemptive value of the two suited hand puts pressure on the opponents to find their fits, games and slams at the three level.
Due to the ease with which it is learned and used, it is among the most abused conventions: the convention is often used by players with insufficient strength or lengths (such as 5-4 in the minors).
This often leads to one of:
- Opponents double and collect a heavy penalty.
- Opponents outbid the bidders to obtain a major suit contract.
- Opponents make a contract that is normally unmakeable due to forewarning of adverse distribution.