In the Indo-European languages, thematic roots are those roots that have a "theme vowel"; a vowel sound that is always present between the root of the word and the attached inflections. Athematic roots lack a theme vowel, and attach their inflections directly to the root itself.

For example, consider the endings of the Latin "first declension" singular:

Nom. rosa
Gen. rosae
Dat. rosae
Acc. rosam
Abl. rosā

The vowel a seems to be prominent in these case endings, so nouns like rosa came to be known as "a-stem" nouns, with a being the "theme vowel," and was later analysed as having a stem containing a root plus a suffix. In fact, philologists now believe that the suffix in Indo-European was now *-eh2, with a laryngeal that usually became a in the daughter languages.

The distinction between thematic and athematic roots is especially apparent in the Greek verb; they fall into two classes that are marked by quite different personal endings. Thematic verbs are also called (-ô) verbs in Greek; athematic verbs are -μι (-mi) verbs, after the first person singular present tense ending that each of them uses. The entire conjugation seems to differ quite markedly between the two sets of verbs, but the differences are really the result of the thematic vowel reacting with the verb endings; in classical Greek, the present tense active endings for athematic verbs are:

-μι, -ς, σι, -μεν, -τε, -ασι(ν)
(-mi, -s, -si, -men, -te, -asi(n))

while the thematic verbs took the endings:

-ω, -εις, -ει, -ομεν, -ετε, -ουσι(ν)
(-ô, -eis, -ei, -omen, -ete, -ousi(n))

In Greek, athematic verbs are a closed class of inherited forms from the parent Indo-European language. Marked contrasts between thematic and athematic forms also appear in Lithuanian, Sanskrit, and Old Church Slavonic. In Latin, almost all verbs are thematic; a handful of surviving athematic forms exist, but they are considered irregular verbs.

The thematic and athematic distinction also applies to nouns; many of the older Indo-European languages distinguish between "vowel stems" and "consonant stems" in the declension of nouns. In Latin, the first, second, fourth, and fifth declensions are vowel stems characterised by a, o, u and e, respectively; the third declension contains both consonant stems and i stems, whose declensions came to closely resemble one another in Latin. Greek, Sanskrit, and other older Indo-European languages also distinguish between vowel and consonant stems, as did Old English.

In modern English, and other languages whose morphology has been drastically simplified by analogy, the distinction between thematic and athematic forms is no longer a meaningful one.

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