- Thematic stem
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In Indo-European linguistics, a thematic stem or vowel stem is a noun or verb stem that ends in a vowel that appears in or otherwise influences the noun or verb's inflectional paradigm. The vowel is called the thematic vowel. The noun or verb is also called thematic.
In Latin, nouns of the first, second, fourth, and fifth declensions are considered thematic; the first declension has the theme vowel a, the second o, the fourth u, and the fifth e. Stems with i are treated together with athematic stems in the third declension. Latin verbs are subject to a similar classification: the first conjugation contains vowel stems with a, the second with e, and the fourth with i. There are no Latin verbs with o or u[citation needed], and very few are athematic (conjugated with consonant stems). Sanskrit, by contrast, preserves a large number of athematic, consonant-stem verbs. Greek resembles both Sanskrit and Latin in different respects.
Most other Indo-European languages have similar distinctions, or had them in their past. In the Germanic languages and the Celtic languages, the theme vowels are often hard to perceive because of the loss of final vowels. However, their presence is still felt, in a manner that defines different ways of declining nouns or conjugating verbs, so philologists still occasionally speak of vowel stems and consonant stems in these languages as well.
See also
Proto-Indo-European language Phonology Accent · Centum-Satem isogloss · Glottalic theory · Laryngeal theory · s-mobile ·
Sound laws (boukólos rule, kʷetwóres rule, Osthoff's, Pinault's, Siebs', Sievers', Stang's, Szemerényi's)Morphology Parts of speech 
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