Appendix:Latin second declension

Description
Latin words of the second declension end in -ī in the genitive singular and end in -us, -r, or -um in the nominative singular. They are usually of masculine or neuter gender.
 * Masculines typically end in -us in the nominative singular (e.g. ).
 * A substantial minority end instead in -r (e.g. ; ; ). Final r here is not a suffix; rather, it comes from the noun's stem.
 * Neuters almost always end in -um in the nominative/accusative singular.
 * Three second-declension neuter nouns have an exception declension with -us in the nominative/accusative singular:, ,.
 * Feminine second-declension nouns take the same endings as the masculine. They are much less common than the masculines and neuters.

Latin words borrowed from Ancient Greek’s second declension are inflected with a varying mixture of Greek and Latin endings.

Examples
1In Archaic Latin, words ending in -ius and -ium take the ending -ī in the genitive singular, e.g. in earlier Roman Republic times, fīlius would change to fīlī in the genitive singular (cf. later and nowadays fīliī), and negōtium would change to negōtī (cf. later and nowadays negōtiī).

2In words ending in -ius, the -ius becomes ī, e.g. fīlius becomes fīlī in the vocative singular.

Examples:
 * mūrus, -ī ; fīlius, -ī
 * ager, agrī ; magister, -trī ;  puer, -erī
 * bellum, -ī ; templum, -ī

N.B.
 * The singular vocative of second declension -us nouns is the only place in pure Latin in which the vocative ever differs from the nominative: -e instead of -us. The plural vocative is the same as the nominative. As seen in filius, filiī, however, the vocative singular changes -ius and -ium nominative endings into an -ī, instead of changing the -us ending into an -e and the repeating the -um ending.
 * deus, -ī  has several irregular plural forms.

Greek declension
Examples:
 * atomus (-os), -ī,
 * ampelos, -ī,
 * mȳthos, -ī,
 * phaenomenon, -ī,

N.B.
 * Genitive, dative, ablative, locative and usually also the plural are the same as in Latin words; for -os/-us it is like Latin -us and words with stem in r/er, and for -on/-um it is like Latin -um.
 * In case of proper nouns and book titles genitive plural -ōn and nominative plural -oe can appear, as in Vergil's Georgicon Libri alias Georgica and in Terence's Adelphoe.