fifthdeclension

The fifth declension is a group of Latin nouns and adjectives that form their characteristic endings with a nominative singular in -ēs and a genitive singular in -eī. It is smaller and more irregular than the other declensions, and its endings show a distinctive pattern: in the singular, -ēs for the nominative, -eī for the genitive and dative, -em for the accusative, and -ē for the ablative; in the plural, -ēs for the nominative and accusative, -ērum for the genitive, and -ēbus for the dative and ablative.

A standard paradigm can be illustrated by the noun rēs, reī, meaning “thing” or “affair.” Singular: N rēs, G rēī, D rēī, A rem, Ab rē. Plural: N rēs, G rērum, D rēbus, A rēs, Ab rēbus. Another well-known member is diēs, diēī, meaning “day,” which uses the same endings but can be masculine or feminine depending on sense. Thus: Singular N diēs, G diēī, D diēī, A diem, Ab diē. Plural N diēs, G diērum, D diēbus, A diēs, Ab diēbus.

Most fifth-declension nouns are feminine, especially the abstract or qualitative kinds, though diēs is a common masculine example when referring to a day. The declension also appears in certain nouns with special meanings or irregulars. Overall, the fifth declension is distinguished by its nominative -ēs and genitive -eī endings, and by its mix of feminine nouns with a few masculine members.