Proto-Indo-European verbs reflect a complex system of morphology, more complicated than the substantive, with verbs categorized according to their aspect, using multiple grammatical moods and voices, and being conjugated according to person, number and tense. In addition to finite forms thus formed, non-finite forms such as participles are also extensively used. The verbal system is clearly represented in Ancient Greek and Vedic Sanskrit, which closely correspond, in nearly all aspects of their verbal systems, and are two of the most well-understood of the early daughter languages of Proto-Indo-European. Basics Verb conjugation in Proto-Indo-European involves the interplay of six dimensions (number, person, voice, mood, aspect and tense) with the following variables identified under the Cowgill-Rix system, which is one of the methodologies proposed and applies only to certain subfamilies: Further, participles can be considered part of the verbal systems although they are not verbs themselves, and as with other PIE nouns, they can be declined across seven or eight cases, for three genders and three numbers. Building blocks Roots The starting point for the morphological analysis of the PIE verb is the root. PIE roots are morphemes with lexical meanings, which usually consist of a single vowel flanked by one or more consonants arranged to very specific rules. Stems and stem formation Before the final endings — to denote number, person, etc., can be applied, additional elements () may be added to the root (). The resulting component here after any such affixion is the stem, to which the final endings () can then be added to obtain the conjugated forms. Athematic and thematic stems Verbs, like nominals, made a basic distinction based on whether a short, ablauting vowel -e- or -o-, called the thematic vowel was affixed to the root before the final endings added. In the case of the thematic conjugations, some of the endings differed depending on whether this vowel was present or absent, but by and large the endings were the same for both types. The athematic system is much older and exhibits ablaut within the paradigm. In the descendant languages, athematic verbs were often extended with a thematic vowel, likely because of the complications resulting from the consonant clusters formed when the mostly consonant-initial endings were added directly onto the mostly consonant-final stems. Consequently, the athematic verbs became a non-productive relic class in the later Indo-European languages. In groups such as Germanic and Italic, the athematic