Page "Fourth Council of Constantinople (Eastern Orthodox)" Paragraph 7

The councils of 869 and 879 represent a break between East and West.

While the previous seven ecumenical councils are recognized as ecumenical and authoritative by both East and West, many Eastern Orthodox Christians recognize the council of 879 as the Eighth Ecumenical Council, arguing that it annulled the earlier one.

This council is referred to as Ecumenical in the Encyclical of the Eastern Patriarchs of 1848.

The Roman Catholic Church, however, recognizes the council of 869 as the Eighth Ecumenical Council and doesn't place the later council among Ecumenical Councils.

At the time that these councils were being held, this division was not entirely apparent.