51 nations have now ratified the legally binding nuclear weapons ban treaty. These parties may no longer develop, test, produce or acquire nuclear weapons, nor may they "assist, encourage or induce" other states to conduct nuclear weapons business. The nine nuclear powers - the USA, China, Russia, France, Great Britain, India, Pakistan, Israel and North Korea - are still not signatories. Tilman Ruff explains why this does not detract from the enormous significance of the agreement for the disarmament of nuclear weapons.