2.4 Fourth declension

Overview of Fourth Declension

Fourth declension nouns share a -u stem and follow a consistent set of endings across all cases. Most are masculine, though a handful of important feminine and neuter nouns belong here too. The key to working with this declension is learning to distinguish it from the second declension, since both have a nominative singular in -us.

Characteristics of Fourth Declension

Stem ending in -u

Every fourth declension noun has a stem ending in -u. This vowel stays constant across all cases and numbers, and it shapes every ending you'll encounter. When you strip the -ūs off the genitive singular, you're left with the -u stem (e.g., portūs → stem portu-).

Gender distribution

The declension is predominantly masculine, but there are notable exceptions you need to memorize:

• Masculine (the majority): senātus (senate), exercitus (army), cursus (course), currus (chariot)
• Feminine: manus (hand), domus (house), idūs (the Ides). These decline exactly like the masculine nouns.
• Neuter: cornū (horn), genū (knee). These have their own distinct endings in the nominative, accusative, and plural.

Full Declension Endings

Here's the complete picture. Refer back to this table as you work through the explanations below.

Nominative Singular Endings

Masculine and feminine nouns

These end in -us: portus (harbor), exercitus (army). This looks identical to the second declension nominative singular (dominus, servus), which is the single biggest source of confusion for beginners. The way you tell them apart is by checking the genitive singular: fourth declension gives you -ūs, second declension gives you -ī.

Neuter nouns

Neuter fourth declension nouns end in -ū: cornū (horn), genū (knee). As with neuters in every declension, the nominative and accusative forms are always identical.

Genitive Singular Ending

All genders end in -ūs (with a long u): portūs (of the harbor), cornūs (of the horn).

This is the form dictionaries give alongside the nominative to identify the declension. When you see a vocabulary entry like portus, -ūs, m., that genitive -ūs tells you it's fourth declension, not second.

Dative Singular Ending

For masculine and feminine nouns, the dative singular ends in -uī, pronounced as three syllables: por-tu-ī. For neuter nouns, the dative singular is -ū (identical to the ablative): cornū.

Some grammars note that the -uī ending occasionally contracts to -ū even for masculine/feminine nouns, but at the elementary level, use -uī as your standard form.

Accusative Singular Endings

Masculine and feminine nouns

These end in -um: portum (harbor). Again, this looks just like the second declension accusative singular. Context and the dictionary entry are your guides.

Neuter nouns

These end in -ū: cornū (horn). Same form as the nominative, following the universal neuter rule.

Ablative Singular Ending

All genders end in -ū (long u): portū (by/with/from the harbor), cornū (by/with/from the horn). Don't confuse this with the second declension ablative singular, which ends in -ō.

Nominative Plural Endings

Masculine and feminine nouns

These end in -ūs: portūs (harbors). Yes, this is the same form as the genitive singular and the accusative plural. You'll need sentence context to tell them apart. This three-way ambiguity is one of the trickiest aspects of fourth declension.

Neuter nouns

These end in -ua: cornua (horns). As always with neuters, the nominative and accusative plural are identical.

Genitive Plural Ending

All genders end in -uum: portuum (of the harbors), cornuum (of the horns). This is two syllables: por-tu-um. The double u is distinctive to this declension and easy to spot.

Dative and Ablative Plural Endings

Both cases end in -ibus for all genders: portibus (to/for the harbors; by/with/from the harbors). This ending is shared with the third declension, so it won't help you distinguish between the two. But the rest of the paradigm will.

Accusative Plural Endings

Masculine and feminine nouns

These end in -ūs: portūs (harbors). Identical to the nominative plural and genitive singular. Only context resolves the ambiguity.

Neuter nouns

These end in -ua: cornua (horns). Identical to the nominative plural, following the standard neuter pattern.

Common Fourth Declension Nouns

These are the words you'll encounter most often:

• senātus, -ūs, m. — senate
• exercitus, -ūs, m. — army
• manus, -ūs, f. — hand, band (of soldiers)
• domus, -ūs, f. — house, home
• currus, -ūs, m. — chariot
• cursus, -ūs, m. — course, running
• cornū, -ūs, n. — horn, wing (of an army)
• genū, -ūs, n. — knee

Many fourth declension nouns are verbal nouns derived from verb supine stems (e.g., adventus from advenīre, conspectus from conspicere). Recognizing this pattern can help you guess meanings.

Exceptions and Irregularities

Domus

Domus (house) is the most important irregular noun in this declension. It mixes fourth and second declension forms:

The locative form domī (at home) appears frequently and is worth memorizing on its own.

Nouns with alternative forms

A few nouns waver between fourth and second declension. Senātus occasionally shows a second declension genitive senātī in some authors. These variants are relatively rare in elementary texts, but be aware they exist when you encounter them in reading.

Fourth Declension vs. Other Declensions

Fourth vs. second declension

This is the comparison that matters most. Both have nominative singular in -us, and both have accusative singular in -um. Here's how to tell them apart:

Fourth vs. third declension

The main overlap is in the dative and ablative plural, where both declensions use -ibus. Beyond that, they differ significantly: third declension nouns have highly varied nominative singular forms and a genitive singular in -is, while fourth declension nouns consistently show -us/-ū in the nominative and -ūs in the genitive.

Exercises and Practice

• Write out the full declension of exercitus and cornū from memory, then check against the table above.
• Given a form like manuum, identify the case(s) and number.
• Translate short sentences that mix fourth and second declension nouns to practice telling them apart.
• Find fourth declension nouns in a Latin passage and parse each one by case and number.