The Treaty on the Limitation of Underground Nuclear Weapon Tests (Aka-the Threshold Test Ban Treaty (or TTBT)) was signed in July 1974 by the US and the USSR.
It establishes a nuclear weapons test blast "threshold," by prohibiting nuclear tests of devices having a atomic yield exceeding 150 kilotons (equivalent to 150,000 tons of TNT).
Of particular significance was the relationship between explosive power of reliable, tested warheads and first-strike capability. Agreement on the Threshold Test Ban Treaty was reached during the summit meeting in Moscow in July 1974.
America and the USSR de facto agreed on 15 December 1984, to a reduced yield of 80 kilotonnes due to the need to get even firther under the limitations proposed by the Threshold Test Ban Treaty.
It came into force on the 11th of December, 1990.