SHAPE: The western group of ancient cities used the X shape, exactly like it happens today with latin alphabet. The eastern type was just like our today's Ξ. (It looks very much like the northsemitic letter, called 'samekh'). The western symbol was passed to the latin alphabet.

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SOUND: Ξ is an innovation: the greeks changed the northsemitic phonetic value to represent a combined double consonant sound: [k]+[s] or [x]+[s] as they did with the monogramme Ψ (psi) for [p]+[s] or [f]+[s]. Also, Z (zeta), Θ (theta), Φ (phi) and X (chi).
When preceded by gamma its pronunciation is affected: ΓΞ [ŋks].

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NAME: ETYMOLOGY: From northsemitic (phoenician) symbol: samekh. The transformation to end up denoting 'khs' is debated.
NAME: SPELLING: The letter's old poloytonic greek orthography was ξε. The EI turned to I: ξ (following the iotacistic tendency).

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COMBINATIONS:when preceded by gamma its pronunciation is affected: ΓΞ [ⁿks].
DOUBLED: No double occurrences.

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MORE GREEK WORDS STARTING WITH XI
•ξένo=foreign,alien - xeno... •ξύλo=wood - xylo...
MORE GREEK WORDS WITH XI
•αξίωμα=axiom