Twelve signatures in Washington and thirty-two members today: the North Atlantic Treaty Organization
Background
The North Atlantic Treaty Organization was created by the North Atlantic Treaty, also called the Washington Treaty, which the Foreign Ministers of twelve countries signed on 4 April 1949 at the Departmental Auditorium in Washington, D.C. The alliance has since grown to 32 member countries. Its headquarters is in Brussels, having earlier been located in Paris. The treaty is a collective defence pact, and the alliance's best known provision is Article 5, under which an attack on one member is treated as an attack on all.
Original signatories:
12 countries
Member countries:
32
Headquarters:
Brussels
Why it matters for exams: The Washington Treaty of 1949, Article 5 on collective defence and NATO's Brussels headquarters are standing questions.
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In plain words
NATO North Atlantic Treaty Organization — A collective defence alliance of North American and European states.