Fourth-Declension Neuter Nouns

The fourth-declension neuter class is tiny but structurally clean. Its central nouns are cornū, cornūs n. “horn; wing of an army,” genū, genūs n. “knee,” and verū, verūs n. “spit, skewer”; pecū, pecūs n. “cattle, herd” is rarer and historically variable. Their nominative singular ends in long -ū, not the short -us described under Fourth-Declension Masculine and Feminine Nouns.

These nouns obey the neuter rule: nominative, accusative, and vocative are identical in each number, and their plural ends in -a. Nominative, dative, accusative, ablative, and even vocative singular of genū look identical, so syntax does more work than the ending. English speakers should identify the Latin construction first and translate second.

The neuter u-stem

The lexical stem ends in u-. In nominative, accusative, and vocative singular, the bare stem appears with long ū: genū. Genitive singular adds -s, producing genūs. Before plural -bus, u normally weakens to i, so the dative and ablative plural are genibus, not genuibus.

Here is the complete paradigm of genū.

The corresponding forms of cornū are cornū, cornūs, cornū, cornū, cornū in the singular and cornua, cornuum, cornibus, cornua, cornibus in the plural. This is a productive paradigm for the few nouns that belong to the class; what is not productive is inventing new fourth-declension neuters from any noun that happens to end in -ū.

These two genū forms are identical. In the first, nominative genū controls dolet; in the second, accusative genū is the object of cūrāvit. English relies on position, while Latin normally allows position to vary and relies here on the surrounding syntax.

One form, several singular cases

Singular genū or cornū may be nominative, dative, accusative, or ablative; it can also be vocative, though direct address to a knee or horn is naturally rare outside personification. Neither macrons nor word order can resolve this syncretism by themselves.

In fasciam genū circumdedit, the verb circumdare permits the thing wrapped to stand in the dative: the bandage is placed around the knee. In cornū signum dedit, “horn” names the instrument and is ablative. In cornū sonuit, it is the subject. Translating every -ū with “with” would destroy the first and third constructions.

Adjectives agree in neuter case, number, and gender but follow their own declension. Thus genū vulnerātum has fourth-declension noun -ū beside second-declension adjective -um. Agreement does not mean identical endings.

The plural in -ua

Neuter plural nominative, accusative, and vocative adds -a directly to the u-stem: genua, cornua, verua. The sequence -ua is two vowels, not an ending -ūa with a long first vowel. Genitive plural is -uum: genuum, cornuum. Dative and ablative plural use -ibus: genibus, cornibus.

The first genua is nominative plural and controls plural dolent; the second is accusative after motion expressed by in; genuum is genitive plural depending on dolōrēs. English marks only “knee/knees” and uses “to” or “of,” whereas Latin distinguishes the relationships through its paradigm and constructions.

Here cornibus is ablative plural of means. It cannot be accusative plural: that form is cornua.

Gender and agreement

Final -ū plus genitive -ūs is the practical dictionary signature of the class. All the core nouns are neuter, so an adjective must be neuter even when English associates the referent with a male person or animal.

The same cornū can denote an animal’s horn or, by spatial extension, an army’s wing. In both senses dextrum is neuter. Meaning does not change the grammatical gender.

The class is small for historical reasons

Fourth-declension neuters are inherited u-stems, but Latin preserved only a handful as ordinary nouns. Cornū, genū, and verū are the regular Classical core. Pecū “cattle” is (archaic/early Latin and poetic) in much of its attestation and belongs to a historically mixed paradigm alongside third-declension pecus, pecoris n. “herd, cattle.” Its plural pecua and dative/ablative pecubus occur, but this lexical history should not be used to remodel cornū or genū.

Ancient grammarians mention a few additional neuter forms, and later technical Latin can preserve or revive rare vocabulary. These are (antiquarian or later learned), not evidence that -ū remained a freely productive noun-forming class in Classical prose.

Parsing and production diagnostics

When reading a form in -ū, retrieve the genitive and gender. If the entry is genū, genūs n., list nominative, dative, accusative, and ablative singular as possibilities. Then test the verb: does it need a subject, direct object, recipient, or instrument? Test any preposition and adjective agreement. Only then choose an English phrasing.

When producing, first assign the noun’s syntactic role. Use genū for every non-genitive singular case; use genūs only for genitive singular. In the plural, apply the ordinary neuter opposition: genua for nominative/accusative/vocative, genuum for genitive, and genibus for dative/ablative.

Common Mistakes

1. Declining genū like a second-declension noun

2. Adding -m to the neuter accusative singular

3. Forming a masculine-looking plural

4. Copying the noun’s ending onto its adjective

5. Keeping u before plural -bus

Key Takeaways

Fourth-declension neuters are a small inherited class identified by nominative -ū, genitive -ūs, and neuter gender. Nominative, dative, accusative, ablative, and vocative singular are syncretic; plural -ua, -uum, and -ibus supplies clearer contrasts. Let syntax resolve the singular form, and make adjectives agree in neuter without copying the noun’s ending.