What you'll notice
You've come through the vestibule, lit your candle, and stepped into the big room, and the first thing you notice is that everything is pointing the same way. The floor runs toward a raised step at the far end, the step runs toward a wall of painted people with three doors in it, and the doors, when they're open, frame a square table under candles. Overhead, if there's a dome, a face the size of a car looks down with a book. There may be pews; there may be open floor with chairs along the walls. Somewhere to the right, or in a loft behind you, people are singing without instruments. The whole room is a road, and the road goes east.
What it's called

Plain words first, then the Church's. The lobby you came through is the narthex. The big room is the nave, which sounds like "navy" because it is: the Latin word for ship. The raised strip in front of the icon wall is the solea, and the bump in its middle, where the Gospel is read, is the ambo (Greek "step up"). The wall of icons is the iconostasis (Greek "icon stand"; Slavonic ikonostas; older Greek templon). Behind it is the sanctuary or, in Slavic usage, the altar, with the Holy Table at its center and a curved back wall, the apse. Hapgood's Service Book walks through all of this in its front matter, "Symbolism of the Church," and that's the list to keep.1

Field Note: the altar isn't the table. In a Catholic or Protestant church "the altar" means the table. In Orthodox usage, especially Slavic, the altar is the whole room behind the icon screen; the table in it is the Holy Table or the Throne (Slavonic prestol). Hapgood's English, oddly, goes the other way: the room is "The Sanctuary (Altar)" and the table is "The Altar (Prestol),"2 and in her rubrics "the Holy Altar" always means the table. So even the books don't agree. Greeks tend to say hieron or "the sanctuary" for the room and hagia trapeza, "holy table," for the table. So when someone says "he went into the altar," he walked through a door, not onto furniture.
What's actually happening
The building is a lesson plan. Saint Germanus of Constantinople, patriarch in the early 700s, explains the parts one at a time: the apse as the cave of Bethlehem and the tomb, the Holy Table as the place of the burial and the throne, the sanctuary as the Holy of Holies, the chancel barrier as the boundary between the sanctuary and the people, the ambo as the stone rolled from the tomb where the angel announced the Resurrection.3 Hapgood, from the Russian side, divides the building into four parts and covers the domes and the standard order of icons on the screen.4 The neat three-room version you'll meet in catechism class, narthex for the world, nave for the Church, sanctuary for the Kingdom, is modern; The Narthex has the older, messier map.
Why it matters
The room is laid out so that you can't stand in it without facing somewhere. Christians have prayed toward the east since before they had buildings, because the Lord was expected from the east like the dawn (Matthew 24:27; Malachi 4:2 (3:20 in the Septuagint)), and Germanus says the custom was handed down by the apostles.5 Once they had buildings, it became a building code (The Courtyard and the Steps has the oldest instruction). Origen, back in the 230s, had already ruled on awkward floor plans: face east anyway, because "the natural is to be set before the artificial."6 The nave is a ship because it's going somewhere. The iconostasis isn't a wall to keep you out; the people painted on it have already crossed over and turned around to look at you (The Iconostasis and Its Doors has that story). A modern Greek catechism walks newcomers through the building the same way, room by room, and treats the floor plan as the first lesson of the faith.7
Where it came from
The nave-and-sanctuary plan is the Roman basilica, the public hall, which the Church took over in the fourth century when it could finally build in daylight. The solea and ambo are from Constantinople; the ambo was once a tall platform in the middle of the nave, joined to the sanctuary by a walkway, and Saint John Chrysostom preached from it.8 The low chancel barrier grew tall over a thousand years into the iconostasis. Greek churches keep the dome and the wide, square nave; Russian churches run longer and often carry several onion domes. American parishes of every background add pews and a lobby.
What you do
If you're Orthodox: stand in the nave, face east, and don't cross the solea unless you're reading or serving. The space behind the screen is for clergy and those blessed to serve; "I just need to grab something" isn't a blessing.
If you're visiting: stand or sit anywhere in the nave; nobody has assigned seats, though the people at the very front have been standing there for thirty years. Don't step up onto the solea or go through any of the doors, even if one is open and a child just did. Everything else is yours to look at.
Go deeper
- Hapgood, Service Book, "Symbolism of the Church," pp. xxvi–xxxiii.
- Saint Germanus of Constantinople, On the Divine Liturgy §§1–11, with Hebrews 9:1–12.
- Apostolic Constitutions II.57 (ANF 7), the oldest building instructions.
- Demetrios Mavropoulos, Passing Through the Church.
Notes
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Hapgood, Service Book, pp. xxvi–xxxiii. ↩
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Hapgood, Service Book, pp. xxvi–xxvii. ↩
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Germanus, On the Divine Liturgy §§4, 7, 9–10. ↩
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Hapgood, Service Book, pp. xxvi–xxvii, xxx–xxxii; compare Mavropoulos, Passing Through the Church, pp. 280–281. ↩
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Germanus, On the Divine Liturgy §11. ↩
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Origen, On Prayer 32 (trans. Curtis; chapter 20 in the CCEL edition). ↩
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Mavropoulos, Passing Through the Church, chapters "The Floor of the Church," "The Soleas," "The Templon / Iconostasion," and "The Holy Sanctuary," pp. 165–306. ↩
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Taft, The Great Entrance, p. 79; Sozomen, Church History VIII.5 (NPNF2 2). ↩
