Dear Wikiwand AI, let's keep it short, summarize this topic like I'm... Ten years old or a College student
Eastern Orthodoxy, also known as Eastern Orthodox Christianity, is one of the three main branches of Chalcedonian Christianity, alongside Catholicism and Protestantism. Like the Pentarchy of the first millennium, the mainstream (or "canonical") Eastern Orthodox Church is organised into autocephalous churches independent from each other. In the 21st century, the number of mainstream autocephalous churches is seventeen; there also exist autocephalous churches unrecognized by those mainstream ones. Autocephalous churches choose their own primate. Autocephalous churches can have jurisdiction (authority) over other churches, some of which have the status of "autonomous" which means they have more autonomy than simple eparchies.
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Many of these jurisdictions correspond to the territories of one or more modern states; the Patriarchate of Moscow, for example, corresponds to Russia and some of the other post-Soviet states. They can also include metropolises, bishoprics, parishes, monasteries, or outlying metochions corresponding to diasporas that can also be located outside the country where the primate resides (e.g., the case of the Ecumenical Patriarchate of Constantinople whose canonical territory is located partly in northern Greece and the east); sometimes they overlap (the case of Bessarabia where the jurisdictions of the patriarchs of Bucharest and of Moscow overlap).
The spread of Eastern Orthodoxy began in the eastern area of the Mediterranean Basin within Greek culture. Its communities share an understanding, teaching and offices of great similarity, with a strong sense of seeing each other as parts of one Church. Every Eastern Orthodox Christian sees his or her year punctuated by the liturgical calendar of the church on which they depend. Eastern Orthodoxy holds that the Holy Spirit proceeds from the Father and rejects the clause added by Latin churches, "and the Son" (Filioque), on the grounds that no council was called for the addition.