In the card game of bridge, the unusual notrump is a conventional bid showing two lower unbid suits.
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 extends 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 convention or a quantitative invitation to 6NT.
The Michaels cuebid is a similar convention which is used to show a two suiter with one or both major suits.
To counteract the unusual notrump, the opening side may employ unusual vs. unusual.
Read more about Unusual Notrump: Modifications, Defense To 2NT (showing Both Minors) As An Over Call and An Opening Bid, Drawbacks
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