Etymology
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*wekw- 
Proto-Indo-European root meaning "to speak."

It forms all or part of: advocate; avocation; calliope; convocation; epic; equivocal; equivocation; evoke; invoke; provoke; revoke; univocal; vocabulary; vocal; vocation; vocative; vociferate; vociferous; voice; vouch; vox; vowel.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit vakti "speaks, says," vacas- "word;" Avestan vac- "speak, say;" Greek eipon (aorist) "spoke, said," epos "word;" Latin vocare "to call," vox "voice, sound, utterance, language, word;" Old Prussian wackis "cry;" German er-wähnen "to mention."
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-i- 
a "connective" element in many words formed with Latin or Greek suffixes, now often felt as part of them (as in -iac, -iacal, -ial, -ian, -ify, -ity, etc.). Properly it forms no proper part of the suffix but is often the stem-vowel of the initial word in the Latin compounds (genial from genius), or a modified form of it. As such forms were very common, -i- was used merely connectively or euphonically in some Latin compounds (uniformis) and in later words made from Latin components in English or French (centennial, editorial).

The Greek equivalent is -o-, which also became an active connective in English, but they now are used indifferently with elements from either language.
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