Analysis of information sources in references of the Wikipedia article "Zeta" in English language version.
Ἔτι δὲ τῶν συμφώνων διπλᾶ μέν ἐστι τρία· ζ ξ ψ. διπλᾶ δὲ εἴρηται, ὅτι ἓν ἕκαστον αὐτῶν ἐκ δύο συμφώνων σύγκειται, τὸ μὲν ζ ἐκ τοῦ σ καὶ δ, τὸ δὲ ξ ἐκ τοῦ κ καὶ σ, τὸ δὲ ψ ἐκ τοῦ π καὶ σ.
Of the consonants, three are double: ζ ξ ψ. They are called double because each one of them is composed of two consonants, ζ of σ and δ; ξ of κ and σ, ψ of π and σ.
— Dionysius Thrax, Περὶ στοιχείου, Τέχνη Γραμματική (~100 BCE).
