X

the twenty-fourth letter of the English alphabet, is borrowed, as to its form, from the Greek X. At the end of words, it has the sound of ks, as in wax, lax; in the middle, the sound of ks, or sometimes of gz, as in axis, example, exhaust. At the beginning of a word, it has the sound of z. See Principles of Pronunciation, § 104.

Before the employment by the Greeks of their character Ξ or ξ, it was their common custom to represent this sound by X Σ rather than by K Σ. The Romans copied this Greek practice, and we consequently find in Latin inscriptions such forms as MAXSVMVS, PROXSVMVS, &c. But the Romans, being generally averse to the aspirated letters, had little or no occasion for the character X, except in this combination with an S. The very sight, therefore, of an X, even before the eye came to the S, raised in the mind the idea of a sibilant, and thus rendered the sibilant itself a superfluous letter, which, because it was superfluous, was before long omitted, and the single letter X made to perform the office of the two consonants X S. Eng. Cyc.