The most prominent feature of an Orthodox church is the Iconostasis, consisting of one or more rows of Icons and broken by a set of doors in the center (the Holy Doors) and a door at each side (the Deacon's Doors). In ancient times, the Iconostasis was probably a screen placed at the extreme Eastern end of the church (a tradition still preserved by Russian Old-Believers), but quite early it was moved out from the wall as a sort of barrier between the Nave and the Altar, with the opening and closing of curtains making the Altar both visible and inaccessible.
The Holy Fathers envisioned the church building as consisting of three mystical parts. According to Patriarch Germanus of Constantinople, a Confessor of Orthodoxy during the iconoclastic controversies (7th-8th Centuries), the church is the earthly heaven where God, Who is above heaven, dwells and abides, and it is more glorious than the [Old Testament] tabernacle of witness. It is foreshadowed in the Patriarchs, is based on the Apostles…, it is foretold by the Prophets, adorned by the Hierarchs, sanctified by the Martyrs, and its high Altar stands firmly founded on their holy remains…. Thus, according to St. Simeon the New Theologian, the [Vestibule] corresponds to earth, the [Nave] to heaven, and the holy [Altar] to what is above heaven [Book on the House of God, Ch. 12].
Following these interpretations, the Iconostasis also has a symbolic meaning. It is seen as the boundary between two worlds: the Divine and the human, the permanent and the transitory. The Holy Icons denote that the Savior, His Mother and the Saints, whom they represent, abide both in Heaven and among men. Thus the Iconostasis both divides the Divine world from the human world, but also unites these same two worlds into one whole a place where all separation is overcome and where reconciliation between God and man is achieved. Standing on the boundary between the Divine and the human, the Iconostasis reveals, by means of its Icons, the ways to this reconciliation.
A typical Iconostasis consists of one or more tiers (rows) of Icons. At the center of the first, or lowest, tier, are the Holy Doors, on which are placed Icons of the four Evangelists who announced to the world the Good News the Gospel of the Savior. At the center of the Holy Doors is an Icon of the Annunciation to the Most-Holy Theotokos, since this event was the prelude or beginning of our salvation. Over the Holy Doors is placed an Icon of the Last Supper since, in the Altar beyond, the Mystery of the Holy Eucharist is celebrated in remembrance of the Savior Who instituted the Sacrament at the Last Supper.
At either side of the Holy Doors are always placed an Icon of the Savior (to the right) and of the Most-Holy Theotokos (to the left). In addition, next to the Icon of the Savior is placed that of the church, i.e., an Icon of the Saint or Event in whose honor the church has been named and dedicated. Other Icons of particular local significance are also placed in this first row, for which reason the lower tier is often called the Local Icons. On either side of the Holy Doors, beyond the Icons of the Lord and His Mother, are two doors Deacon's Doors upon which are depicted either sainted Deacons or Angels who minister always at the heavenly Altar, just as do the earthly Deacons during the Divine services.
Ascending above the Local Icons are several more rows (or tiers) of Icons. The tier immediately above are those representing the principal Feasts of the Lord and the Theotokos. The next tier above that contains Icons of those Saints closest to the Savior, usually the Holy Apostles. Just above the Icon of the Last Supper is placed an Icon of the Savior in royal garments, flanked by His Mother and St. John the Baptist, called the Deisis (prayer), since the Theotokos and the Forerunner are turned to Him in supplication. As these Icons (Apostles, Theotokos, and Forerunner) are arranged in order on either side of the Savior the tier is usually called the Tchin (or rank). Often this tier was to be found just above the Local Icons and below the Feast Day Icons.
The next row usually contains the Old Testament Saints Prophets, Kings, etc. in the midst of which is the Birthgiver of God with the Divine Infant Who is from everlasting and Who was their hope, their consolation, and the subject of their prophecies. If there are more tiers, Icons of the Martyrs and Holy Bishops would be placed above the Old Testament Saints. At the very top of the Iconostasis is placed the Holy Cross, upon which the Lord was crucified, effecting thereby our salvation.
As pointed out, the central place of the Iconostasis is occupied by the Holy Doors, because the Mystery of the Holy Eucharist celebrated within the Altar, is brought forth through them to the faithful. They are also called the Royal Gates (or Doors), since the King of Glory passes through them in the Holy Eucharist. Behind the doors is placed a curtain which is opened or closed, depending on the solemnity or penitential aspect of a particular moment of the Divine services.