Quārta et Quīnta Dēclīnātiō

The Fourth Declension consists of nouns whose stem ends in -u-. The genitive singular ends in -ūs; this ending is the diagnostic sign of the declension and distinguishes 4th-decl. nouns from 2nd-decl. masculines (which also end in -us in the nominative singular but have gen. -ī).

Most nouns of this declension are masculine, ending in -us in the nominative singular. Common examples:
• frūctus, frūctūs (m.) "fruit, profit, enjoyment".
• exercitus, exercitūs (m.) "army".
• portus, portūs (m.) "harbor".
• cāsus, cāsūs (m.) "case, fall, accident".
• senātus, senātūs (m.) "senate".
• magistrātus, magistrātūs (m.) "magistracy, magistrate".
• versus, versūs (m.) "verse".
• cantus, cantūs (m.) "song".
• cōnspectus, cōnspectūs (m.) "view, sight".
• cursus, cursūs (m.) "course, race".

A few are neuter, ending in -ū in the nominative singular:
• cornū, cornūs (n.) "horn".
• genū, genūs (n.) "knee".
• verū, verūs (n.) "spit (for cooking)".
• tonitrū, tonitrūs (n.) "thunder".

The case-endings of the fourth declension derive from the contraction of stem-vowel u- with the regular endings:

Singular masculine: -us, -ūs, -uī, -um, -ū (Nōm./Voc., Gen., Dat., Acc., Abl.).
Plural masculine: -ūs, -uum, -ibus, -ūs, -ibus.
Singular neuter: -ū, -ūs, -ū, -ū, -ū (Nōm./Acc./Voc., Gen., Dat., Abl. — all but Gen. share form).
Plural neuter: -ua, -uum, -ibus, -ua, -ibus.

The 4th declension is the smallest of Latin's five — perhaps 200 nouns total. But several are very high-frequency (especially frūctus, exercitus, senātus, manus, domus, diēs... wait, the last two are 4th and 5th respectively). The declension is sometimes called the "u-declension" (dēclīnātiō ū-stēmum) for clarity.

The general rule for gender:

• Nouns of the fourth declension in -us are masculine.
• Those in -ū are neuter.

The chief feminine exceptions (mostly trees, plants, names, and a few common words):

• manus, manūs (f.) "hand" — extremely high-frequency.
• domus, domūs (f.) "house, home" — irregular, mixed with 2nd decl. (see § 93).
• tribus, tribūs (f.) "tribe".
• porticus, porticūs (f.) "portico, colonnade".
• īdūs, īduum (f.; only plural) "the Ides (of the month)".
• quercus, quercūs (f.) "oak".
• acus, acūs (f.) "needle".
• nurus, nurūs (f.) "daughter-in-law".
• socrus, socrūs (f.) "mother-in-law".
• Names of trees and plants generally: laurus, pīnus, populus (poplar), cupressus (cypress).

The fact that "tree-names are feminine" reflects an ancient gendering of plants as feminine (as nurturers/bearers); it is consistent across many Indo-European languages.

A few words have collateral forms in the second declension: laurus / laurī (laurel), pīnus / pīnī (pine). Both 4th and 2nd-decl. forms appear in classical authors, often in the same author at different points.

The noun domus (f.) "house" is one of the most peculiar in Latin morphology — it has forms drawn from both the second and fourth declensions, with strong preferences in classical usage. The dual origin reflects the noun's Indo-European past (cognate with Greek domos), where it began in a different declension and was partially attracted to a second.

The forms most commonly used in classical prose are shown below. Note especially:

• The locative domī "at home" — preserved as a fossil from the older system. It serves as the regular way to say "at home" in Latin (as opposed to in domō, which would be unusual).

• The accusative domum "homewards" — used WITHOUT preposition with verbs of motion. This contrasts with most accusatives, which require ad or in.

• The ablative domō "from home" — used WITHOUT preposition with verbs of departure. Similar idiomatic use.

These three locative-like uses (domī, domum, domō) parallel the use of city-names (Rōmae, Rōmam, Rōmā) — they form a small group of Latin nouns that retain ancient locative-directional senses without prepositions.

The Fifth Declension is the smallest of Latin's declensions — perhaps 30 nouns, of which only a handful are common. It consists of nouns whose stem ends in -ē-. The genitive singular ends in -ēī (after a vowel) or -eī (after a consonant); this is the diagnostic marker. The nominative singular always ends in -ēs.

All nouns of this declension are feminine, with two exceptions:

• diēs, diēī "day" — masculine in classical singular, sometimes feminine in archaic poetry; always masculine in plural.
• merīdiēs, merīdiēī (m.) "midday, noon" — derived from diēs.

Most nouns are abstract nouns (mental or moral concepts):
• rēs, reī (f.) "thing, matter, affair, business" — extremely high-frequency.
• spēs, speī (f.) "hope".
• fidēs, fideī (f.) "faith, trust".
• aciēs, aciēī (f.) "battle line, edge of a sword, sharp point".
• faciēs, faciēī (f.) "face, appearance".
• speciēs, speciēī (f.) "kind, appearance, look".
• seriēs, seriēī (f.) "series, succession".
• diēs, diēī (m./f.) "day".
• rabiēs, rabiēī (f.) "madness, rage".
• scabiēs, scabiēī (f.) "itch, mange".

Most nouns of the fifth declension are defective in the plural: only diēs and rēs are fully declined in the plural. The others have only nominative and accusative plural at most. This defectivity is partly because abstract nouns rarely need plurals.

Defective nouns (nōmina dēfectīva) lack one or more cases or numbers. Some categories:

(a) Plural-only (plūrāle tantum): nouns existing only in the plural (often collective or pair-like in sense):
• arma, armōrum (n.pl.) "arms, weapons".
• cāstra, cāstrōrum (n.pl.) "camp" (military).
• līberī, līberōrum (m.pl.) "children" (one's own).
• moenia, moenium (n.pl.) "city walls".
• nuptiae, nuptiārum (f.pl.) "wedding".
• īnsidiae, īnsidiārum (f.pl.) "ambush".
• fastī, fastōrum (m.pl.) "calendar, fasti (record of office-holders)".

(b) Singular-only (singulāre tantum): nouns existing only in the singular: most uncountable nouns and proper names. aurum "gold" (rarely pluralized for "gold pieces"); iūstitia "justice"; many gods' names.

(c) Specific case missing: vīs "force" lacks gen. and dat. sg. in classical use; iter has alternating stems.

Variable nouns (nōmina variābilia) change gender or declension between singular and plural:
• locus, -ī (m.) "place" → in the plural sometimes loca (n.pl.) "regions, scattered places" but locī (m.pl.) "passages, individual spots".
• iocus, -ī (m.) "joke" → ioca (n.pl.) or iocī (m.pl.).
• caelum, -ī (n.) "heaven, sky" → lacks regular plural; rare caelī in Christian Latin.

Personal names (nōmina hominum) follow the formula praenōmen + nōmen + cognōmen: Mārcus Tullius Cicerō. The praenōmen is normally abbreviated:
• M. = Mārcus.
• P. = Pūblius.
• C. = Gāius (note the spelling: pronounced "Gaius").
• L. = Lūcius.
• Q. = Quīntus.
• T. = Titus.
• Cn. = Gnaeus (note: pronounced "Cnaeus" or "Gnaeus").
• Sex. = Sextus.
• Sp. = Spurius.

Women in the historical period were named with the feminine of the gens (clan): Cornēlia (of the Cornēliī), Iūlia, Tullia. Daughters were distinguished by adjectives such as maior (the elder), minor (the younger), or by ordinals (prīma, secunda, tertia).