In the card game of bridge, the unusual notrump is a conventional showing a two-suited hand. It was originally devised by Al Roth in 1948 with Tobias Stone, to show the minor suits after the opponents opened in a major.
The convention concept is now generally extended to show the "two lowest unbid" suits. Thus, over 1, 2NT shows diamonds and hearts; and over 1, it shows clubs and hearts. In addition to succinctly describing the hand to the partner, it deprives the opponents of bidding space, i.e. it has a preemptive effect. A drawback is that it provides the opponents with information about the strength and distribution of the overcaller's hand, potentially enabling improved declarer play should they secure the contract.
History
In his seminal 1953 book, The Roth-Stone System, Alvin Roth suggested that poor matchpoint scores usually result when the opponents are permitted to play low-level contracts and recommended protective bidding tactics for the player in the pass-out (balancing) seat in such auctions.
He advocated use of weak jump overcalls and responses at a time when the standard practice was for jump overcalls and responses to show strong hands.