Note: Do not rely on this information. It is very old.
Z, the last letter of the English alphabet, was the sixth letter in the Greek alphabet, where it is thought to have had the force of ts or ds. In Italian it still has its force, and is sometimes doubled, and represents the Latin di or ti. Z was introduced into Latin in the time of Cicero, and was apparently confined to words derived from Greek. In German it sounds as ts, in Spanish as th.