3rddeclension

In Latin grammar, the third declension is a large and varied class of nouns. It includes masculine, feminine, and neuter nouns and is defined chiefly by the genitive singular ending -is. The nominative singular form varies widely from word to word, unlike the regular endings of the first two declensions.

The third declension contains consonant-stem nouns and a subset known as i-stem nouns. The hallmark is genitive singular -is. Singular endings for masculine and feminine nouns are: Nominative — varies; Genitive -is; Dative -i; Accusative -em; Ablative -e. Plural endings: Nominative -es; Genitive -um; Dative -ibus; Accusative -es; Ablative -ibus. For neuter nouns, singular genitive -is; Dative -i; nominative and accusative share the same form; Ablative -e. In the neuter plural, nominative and accusative end in -a; genitive -um; dative -ibus; ablative -ibus. A subset known as i-stem nouns shows some deviations, notably a genitive plural in -ium and related alternations.

Examples illustrate typical patterns. Pater, patris, patri, patrem, patre; patres, patrum, patribus, patres, patribus illustrate the masculine/feminine paradigm. Corpus, corporis, corpori, corpus, corpore; corpora, corporum, corporibus, corpora, corporibus illustrate the neuter paradigm. Some i-stem nouns show deviations such as a genitive plural in -ium.

Because of its breadth and irregularities, the third declension is the most common noun class in Latin, producing a large portion of the vocabulary and many adjectives in the third declension as well as numerous borrowings.