United Nations System
There are 15 members of the Security Council, consisting of five veto-wielding permanent members (China, France, Russia, the United Kingdom, and the United States) and 10 elected non-permanent members with two-year terms. This basic structure is set out in Chapter V of the UN Charter. Security Council members must always be present at UN headquarters in New York so that the Security Council can meet at any time.
Supermajority
The United Nations Security Council requires a supermajority of the fixed membership on substantive matters (procedural matters require a simple majority of those present and voting). According to Article 27 of the United Nations Charter, at least nine of the Security Council's 15 members (i.e., a three-fifths supermajority) must vote in favor of a draft resolution in order to achieve passage. Specifying the fixed membership has the effect of making abstentions count as votes against—absences are not normal but would be treated the same way.