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How many letters are in the Classical Greek alphabet?

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Thematic map of Eurasia showing the approximate geographic distribution of Indo-European language families, each represented by a different color. Areas include Celtic in western Europe, Germanic in northern and central Europe, Romance in southwestern Europe, Slavic in eastern Europe and Russia, and Indo-Iranian languages in South Asia. Non-Indo-European language regions and areas of common bilingualism are also indicated. — Greek language Approximate locations of Indo-European languages in contemporary Eurasia.

The Classical Greek alphabet consists of 24 letters.12

Following the official adoption of the Ionic alphabet by Athens in 403 BCE, this 24-letter system became the standard Greek alphabet by the middle of the 4th century BCE, replacing various local scripts.132