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Zeta (uppercase ?, lowercase ?) is the sixth letter of the Greek alphabet. In the system of Greek numerals it has a value of 7. It was derived from the Phoenician letter Zayin. Letters that arose from Zeta include the Roman Z and Cyrillic ? (Ze).
Unlike the other Greek letters, this letter did not take its name from the Phoenician letter it was derived from - it was given a new name on the pattern of eta and theta. The reason the number associated with this letter doesn't match its alphabetical order is because the archaic letter Digamma used to be the sixth letter, and thus Zeta was the seventh letter.