Third Declension Nouns: Family Overview
Third declension is Latin's biggest, messiest noun family — and the one that hides its identity in the nominative.
rēx, flūmen, cīvis, mare, virtūs, iter all live here, each with a nominative that looks nothing like the rest of its paradigm.
The trick of survival is to ignore the nominative and learn the genitive stem: rēx, rēg-is; flūmen, flūmin-is; cīvis, cīv-is. Once you have the stem, the endings are uniform across the family.
The family splits two ways. Consonant stems (most masculine and feminine, plus most neuters) take -um in the gen. pl. and -e in the abl. sg. I-stems — some m/f and most neuters in -e, -al, -ar — slip an i into the gen. pl. (-ium), the neuter nom./acc. pl. (-ia), and often the abl. sg. (-ī).
Spotting which side a noun is on is the whole game.
One set of endings on the genitive stem; consonant stems take -um, i-stems take -ium in the genitive plural.
The nominative is unpredictable. Learn each noun as nominative + genitive — the genitive minus -is is the working stem.
| Case | Singular | Plural |
|---|---|---|
| Nom. | rēx | rēgēs |
| Gen. | rēgis | rēgum |
| Dat. | rēgī | rēgibus |
| Acc. | rēgem | rēgēs |
| Abl. | rēge | rēgibus |
| Case | Singular | Plural |
|---|---|---|
| Nom. | flūmen | flūmina |
| Gen. | flūminis | flūminum |
| Dat. | flūminī | flūminibus |
| Acc. | flūmen | flūmina |
| Abl. | flūmine | flūminibus |
| Case | Singular | Plural |
|---|---|---|
| Nom. | cīvis | cīvēs |
| Gen. | cīvis | cīvium |
| Dat. | cīvī | cīvibus |
| Acc. | cīvem | cīvīs (-ēs) |
| Abl. | cīve | cīvibus |
| Case | Singular | Plural |
|---|---|---|
| Nom. | mare | maria |
| Gen. | maris | marium |
| Dat. | marī | maribus |
| Acc. | mare | maria |
| Abl. | marī | maribus |
See It In Action
— Caes. B. G. iii. 14
Three third-declension forms in one clause: colles (mixed i-stem, nom. pl.), mare (neuter i-stem, acc. sg.), and despectus sneaking in as 4th declension — the same case endings, different family.
— Caes. B. G. i. 10
iter, itineris is a textbook irregular: the nominative comes from a short stem (iter-), but every other case uses the longer itiner-. Always look up the genitive.
— Caes. B. G. i. 13
virtūte — abl. sg. ending in -e. virtūs is a consonant stem (gen. virtūtis), so it follows the consonant pattern; an i-stem here would have virtūtī.
— Caes. B. G. i. 47
Five third-declension nouns in one breath — virtūte, hūmānitāte, adulēscentem, pater, cīvitāte. They all hide their stems in the nominative; the abl./acc. forms reveal them.
Both look like 3rd declension in the nominative. The split shows up in the genitive plural and a few other corners.
gen. pl. -um, abl. sg. -e, neuter pl. -a
rēx, rēgis → rēgum
of the kings (no i)
gen. pl. -ium, abl. sg. often -ī, neuter pl. -ia
cīvis, cīvis → cīvium
of the citizens (i-stem signal)
Tip: Look at the gen. pl. first. -ium = i-stem; -um = consonant stem. Then check the noun's profile (parisyllabic? monosyllable in -s/-x after consonant? neuter in -e/-al/-ar?) — those predict which side it's on.
You meet cīvium in a sentence. What does the -ium tell you, and what would the consonant-stem equivalent look like?
Study Tips
- •Always learn third-declension nouns as a pair: nominative + genitive (rēx, rēgis; flūmen, flūminis). The genitive gives you the stem the rest of the paradigm hangs on.
- •When you see -ium in the genitive plural or -ia in a neuter nom./acc. plural, you're inside an i-stem. Memorize the four i-stem signals (gen. pl. -ium, neuter pl. -ia, abl. sg. -ī, acc. sg. -im) — they're how the textbook will test you.
- •Don't try to predict gender from the nominative ending alone — virtūs (F.) and corpus (N.) both end in -s. Learn each noun's gender with its dictionary entry; the rules in §§ 85–87 are tendencies with real exceptions.