Ξ (X)
Ξ is an Ancient Greek form of ἐκ, meaning “out of, from (+gen.)”. It is the preposition.
Found 123 times in the Ancient Greek texts on Socratos.
Where it is used
Euripides, Iphigenia in Tauris 208: τᾷ μναστευθείσᾳ ʼξ Ἑλλάνων,
Aeschylus, Supplices 10: ʼξʼονοταζόμεναι.
Aristotle, Poetics 21.22: ἄρρενα μὲν ὅσα τελευτᾷ εἰς τὸ Ν καὶ Ρ καὶ Σ καὶ ὅσα ἐκ τούτου σύγκειται (ταῦτα δ’ ἐστὶν δύο, Ψ καὶ Ξ), θήλεα δὲ ὅσα ἐκ τῶν φωνηέντων εἴς τε τὰ ἀεὶ μακρά,
Source: Socratos (socratos.org), a free reader for Ancient Greek and Latin. Grammar from the Perseus Treebank and Morpheus; dictionary entries from Liddell-Scott-Jones, A Greek-English Lexicon (Perseus Digital Library, CC BY-SA). Counts are from the texts on Socratos.