By the first of November it was done: a text agreed, the signatures collected, several hundred men going home before the weather turned. What they'd settled ran to about a paragraph — one Christ, in two natures, without confusion, without change, without division, without separation — and a great deal of the argument had turned on whether you said from two natures or in two natures. Ek or en. Weeks, on a preposition.
It opened in the church of St Euphemia at Chalcedon, just across the water from Constantinople, in 451, convoked by the emperor Marcian. Over five hundred and twenty bishops came, or sent someone to stand in for them, which makes it the largest of the early councils and much the best documented: imperial officials sat in the middle to keep order and to keep minutes, so we can still read people interrupting each other, calling for a document to be read out again, objecting to how a previous session had been recorded. Somebody fed all of them for three and a half weeks. Nobody wrote that part down.
And it didn't settle everything. The Coptic, Syriac, Armenian and Ethiopian churches didn't receive the definition, holding to one incarnate nature of the Word — not, as they have always said, a denial of Christ's humanity. Theologians from both sides have spent the last several decades in conversation about whether the disagreement was ever as wide as the words made it look; they don't all agree on the answer.
Evagrius describes the Euphemia complex as three parts — an open court with colonnades, then a hall, then a round domed chamber where the relics were kept, up on the slope with the water below. Five hundred bishops in a building made for pilgrims arriving a few at a time. Somebody had to find them all somewhere to sit.
Miriam, the bookish afterlife of that preposition is its own long story: the Acts survive in Greek and in a Latin version revised in the sixth century by the deacon Rusticus, who went round Constantinople collating copies because he didn't trust the ones he had, and left notes saying as much. Weeks on ek or en, then centuries on how to render it.