The Mercosur - Mercado Común del Sur /Southern Common Market - is the fourth largest trade bloc in the world in order of economic importance. It is made up of four countries: Argentina, Brazil,Paraguay and Uruguay. The Mercosur was established following the Treaty of Asuncion, signed on 26 March 1991. The definitive form adopted by the bloc arose on the basis of the Ouro Preto Protocols, agreed in December 1994. On 1 January 1995 a Customs Union was born between the four signatory countries.