The number of non-permanent members of the UN Security Council was expanded from 11 to 15 in the year :
(1) 1945
(2) 1955
(3) 1965
(4) 1975

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The membership of the UN Security Council was expanded from 11 to 15 in 1965.

The United Nations (UN) has six main organs. Five of them — the General Assembly, the Security Council, the Economic and Social Council, the Trusteeship Council and the Secretariat — are based at UN Headquarters in New York. The sixth, the International Court of Justice, is located at The Hague in the Netherlands.
In the Security Council, there are five permanent members and ten non-permanent members. The Charter gave the permanent members a privileged position to bring about stability in the world after the Second World War. The main privileges of the five permanent members are permanency and the veto power. The non-permanent members serve for only two years at a time and give way after that period to newly elected members. A country cannot be re-elected immediately after completing a term of two years. The non-permanent members are elected in a manner so that they represent all continents of the world.