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Thethirddeclension is a category ofnouns inLatin andGreek with broadly similarcase formation — diversestems, but similarendings.Sanskrit also has a corresponding class (although not commonly termed asthird), in which the so-calledbasic case endings are applied very regularly.
In contrast with thefirst- andsecond-declension endings, those of the third declension lack atheme vowel (a or o/u in the first and second declensions) and so are calledathematic.
One distinguishing feature of third-declension nouns is agenitivesingular ending of a short vowel ands: Latinrēg-is "of a king" Greek χειρ-ός (cheir-ós) "of a hand", and Sanskritbhagavat-as "of the blessed (one)". Another is adative singular ending ofi (shorti in Greek, longī in Latin):rēg-ī "for a king"; χειρ-ί (cheir-í) "for, with the hand". This corresponds to an-e ending in Sanskrit, which might have been a contractedai or lengthenedi:bhagavat-e "for the blessed (one)"
Many third-declension nouns, unlike first- or second-declension nouns, show different stems depending on case and number — usually one stem for thenominative singular, and another for the rest of the cases, though some Greek nouns have three stems. Greek stems are often formed byablaut: Latinhomō "person" andhomin-ēs "people"; Greekπατήρ (patēr’) "father", πατρ-ός (patr-ós) "of a father", and πατέρ-ες (patér-es), "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 withconsonant 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: Latinrēx "king", fromrēg-s (compare the earlier-mentionedrēgis);πούς (poús) "foot", and Attic dative plural ποσί (posí) "on foot" from πόδ-ς (pód-s) and ποδ-σί (pod-sí). These changes are subject tosandhi in Sanskrit.
Other Greek nouns whose stems in the earliest Greek (notablyMycenaean) ended in ι (i) or υ (u), andj (English consonantaly) orϝ (digamma; Englishw) ine-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 bycompensatory lengthening. InAttic, if there is a short vowel adjacent in the ending, the two vowels switch their lengths byquantitative metathesis. Illustrative of the process is the development of the genitive singular ofβασιλεύς (basileús), "king",πόλις (pólis), "city", andἄστυ (ástu), "town":