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Greek origins.

Prefix epi-

Root word calyx, emphasis typically on the first syllable cal- followed by y as a short i vowel sound, and x as a hard -cks consonant like exit instead of being slurred into a z sound like xylophone.

Fun with English!

Thank you, that is a good explanation, I still have trouble with yx, xy as in xylophone makes more sense to me, but it is what it is.