The instruction Inter oecumenici of 26 September 1964, issued by the Sacred Congregation of Rites while the Council was still in session, and coming into effect on 7 March 1965 made significant changes to the existing liturgy, though the form of the rite was substantially preserved.

The instruction Inter Oecumenici of 26 September 1964 initiated the application to the Mass of the decisions that the Council had taken less than a year before.

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