Threshold Test Ban Treaty

The Treaty on the Limitation of Underground Nuclear Weapon Tests, also known as the Threshold Test Ban Treaty (TTBT), was signed in July 1974 by the United States and Soviet Union. It establishes a nuclear "threshold" by prohibiting nuclear tests of devices having a yield exceeding 150 kilotons after March 31, 1976.

Source: Wikipedia — Threshold Test Ban Treaty (CC BY-SA 4.0)