The third declension is a category of nouns in Latin and Greek with broadly similar caseformation — diverse stems, but similar endings. Sanskrit also has a corresponding class, in which the so-calledbasic case endings are applied very regularly. In contrast with the first- and second-declension endings, those of the third declension lack a theme vowel and so are called athematic. One distinguishing feature of third-declension nouns is a genitivesingular ending of a short vowel and s: Latin rēg-is "of a king" Greek χειρ-ός "of a hand", and Sanskrit bhagavat-as "of the blessed ". Another is a dative singular ending of i : rēg-ī "for a king"; χειρ-ί "for, with the hand". This corresponds to an -e ending in Sanskrit, which might have been a contracted ai or lengthened i: bhagavat-e "for the blessed " Many third-declension nouns, unlike first- or second-declension nouns, show different stems depending on case and number — usually one stem for the nominative singular, and another for the rest of the cases, though some Greek nouns have three stems. Greek stems are often formed by ablaut: Latin ' "person" and homin-ēs "people"; Greek "father", πατρ-ός "of a father", and πατέρ-ες, "fathers". In Sanskrit the situation is similar to that in Greek, but the strongest stem is used somewhat more. A subcategory within both the Latin and Greek third declension is nouns with consonant stems. These, unlike all first- and second-declension nouns, end in a consonant. Often the consonant at the beginning of certain endings undergoes a sound change with the consonant of the stem: Latin ' "king", from rēg-s ; "foot", and Attic dative plural ποσί "on foot" from πόδ-ς and ποδ-σί. These changes are subject to sandhi in Sanskrit.
Greek third-declension nouns with vowel endings
Other Greek nouns whose stems in the earliest Greek ended in ι or υ, and j or ϝ in e-grade, have in later Greek undergone sound changes that markedly distinguish them from run-of-the-mill third-declension nouns. In particular, the stems with j or ϝ lose this sound, and in some cases the preceding vowel is lengthened by compensatory lengthening. In Attic, if there is a short vowel adjacent in the ending, the two vowelsswitch their lengths by quantitative metathesis. Illustrative of the process is the development of the genitive singular of , "king", , "city", and , "town":
