The Latin X's descent from Greek chi is covered elsewhere in this atlas, but the relationship is more complicated than a simple borrowing. Classical Athenian Greek used chi for an aspirated /kh/ sound, but the western Greek alphabets of southern Italy, the ones Etruscan and Rome actually drew their letters from, used the same X-shaped letter for the very different consonant cluster /ks/, the value Latin inherited and English still uses. Because Etruscan and early Latin already had other letters covering the sounds Athenian chi represented, X entered the Roman alphabet carrying only its western Greek value, and Latin placed it near the end of its native letter sequence, roughly corresponding to the position once held by the Phoenician letter samekh, which classical Greek instead used for its own letter xi.
The Roman capital X, two crossed diagonal strokes, has remained essentially unchanged since antiquity, and it does double duty, like I, V, L, C, D and M, as a Roman numeral: X stands for ten, a value the standard account derives, like I and V, from an ancient tally notch rather than from the alphabetic letter's own history. That numeral use, quite apart from the letter's phonetic role, is also why a mark meant to stand in for an unknown or unwritten signature is conventionally an X, a habit going back at least to medieval documents signed by people who could not write their own name.