2.3 Third declension

Overview of third declension

Third declension is the largest and most varied noun group in Latin. Unlike first and second declension, where you can usually predict the forms from the nominative, third declension nouns change stems between the nominative and other cases. That means you must know the genitive singular to work with these nouns.

There are three sub-types within third declension: consonant stems, i-stems, and mixed stems. Each follows slightly different ending patterns, so recognizing which type you're dealing with is a core skill for this unit.

Stem types

Consonant stems

These are the most common third declension nouns. Their stems end in a consonant (like b, c, d, g, l, n, r, s, t), and you find the stem by dropping the -is from the genitive singular.

The nominative singular often looks very different from the stem. For example, rex ("king") has the genitive regis, so the stem is reg-. Similarly, consul ("consul") has the genitive consulis, giving you the stem consul-. Always check the genitive form in your vocabulary list.

I-stems

I-stems have an -i- that appears before certain case endings. Three categories of nouns qualify as i-stems:

• Equal-syllable nouns: masculine and feminine nouns ending in -is or -es where the nominative and genitive have the same number of syllables (e.g., nubes, nubis, "cloud"; civis, civis, "citizen")
• Parisyllabic-base nouns: masculine and feminine nouns whose stem (found from the genitive) ends in two consonants (e.g., urbs, urbis, "city"; mons, montis, "mountain")
• Neuter nouns ending in -e, -al, or -ar (e.g., mare, maris, "sea"; animal, animalis, "animal"; exemplar, exemplaris, "example")

The i-stem distinction matters most in the genitive plural (-ium instead of -um), the neuter nominative/accusative plural (-ia instead of -a), and sometimes the ablative singular (-ī instead of -e).

Mixed stems

Mixed stems blend features of consonant stems and i-stems. They typically use -ium in the genitive plural (like i-stems) but -e in the ablative singular (like consonant stems). A noun like civitas, civitatis ("state") follows this pattern. You'll need to learn which nouns behave this way on a word-by-word basis, though the categories above give you a starting framework.

Gender in third declension

Gender in third declension is less predictable than in first or second declension, so you should memorize the gender of each noun as you learn it. That said, some patterns help.

Masculine nouns

Common nominative endings for masculines include -or, -os, -er, and -es (with irregular vowel length). Examples:

• labor, laboris (work)
• mos, moris (custom)
• pater, patris (father)
• pes, pedis (foot)

Feminine nouns

Typical feminine endings include -as, -es, -is, -x, and -us (when the genitive is -utis or -udis). Examples:

• civitas, civitatis (state)
• nubes, nubis (cloud)
• navis, navis (ship)
• pax, pacis (peace)
• virtus, virtutis (courage)

Abstract nouns are often feminine: libertas (freedom), veritas (truth).

Neuter nouns

Neuter endings include -us (genitive -oris or -eris), -men, -e, -al, and -ar. Examples:

• tempus, temporis (time)
• nomen, nominis (name)
• corpus, corporis (body)

Two rules for neuters that never change across any declension:

• Nominative and accusative forms are always identical.
• The nominative/accusative plural always ends in -a (or -ia for i-stems).

Case endings

Here's where the real work of third declension happens. The table below shows the standard endings for consonant stems and i-stems side by side.

Singular endings

For the ablative singular, consonant stems use -e. Neuter i-stems (like mare) consistently use -ī. Masculine and feminine i-stems can sometimes use either -e or -ī, though -e is more common in prose.

Plural endings

1. Genitive plural: -um vs. -ium
2. Neuter nom./acc. plural: -a vs. -ia
3. Masc./fem. accusative plural: i-stems sometimes use -īs instead of -ēs

The dative and ablative plural ending -ibus is the same for all third declension nouns, regardless of stem type.

I-stem vs. consonant stem

Telling these apart is one of the trickiest parts of third declension. Here's a quick identification checklist:

A noun is likely an i-stem if it meets any of these criteria:

• Masculine or feminine with the same number of syllables in nominative and genitive (e.g., civis, civis)
• Masculine or feminine with a stem ending in two consonants (e.g., mons, mont-is, stem mont-)
• Neuter ending in -e, -al, or -ar

If none of these apply, treat it as a consonant stem.

Where the difference shows up

• Genitive plural: consonant stems take -um (e.g., regum); i-stems take -ium (e.g., civium)
• Neuter nom./acc. plural: consonant stems take -a (e.g., corpora); i-stems take -ia (e.g., maria)
• Ablative singular: consonant stems take -e; neuter i-stems take -ī; masculine/feminine i-stems usually take -e but sometimes -ī

Irregular nouns

Several high-frequency third declension nouns have irregular forms. These just need to be memorized.

• vīs (force): highly irregular in the singular (vīs, —, —, vim, vī), but regular i-stem forms in the plural (vīrēs, vīrium, vīribus, vīrēs/vīrīs, vīribus). Note the stem change from vi- to vīr-.
• iter, itineris (journey): the stem changes to itiner- in all cases except the nominative and accusative singular.
• bōs, bovis (ox, cow): has a unique stem bov- and some unusual forms (boum in genitive plural, bōbus or būbus in dative/ablative plural).
• Iuppiter (Jupiter): the oblique cases use the stem Iov- (Iovis, Iovī, Iovem, Iove).

Third declension adjectives

Third declension adjectives follow i-stem patterns. They're classified by how many distinct forms they have in the nominative singular.

Three-termination adjectives

These have separate forms for masculine, feminine, and neuter in the nominative singular. The pattern is -er, -ris, -re:

• ācer, ācris, ācre (sharp, keen)
• celer, celeris, celere (swift)

The masculine form drops the -e- in some adjectives (like ācer) but keeps it in others (like celer). Check your vocabulary. All other cases decline like regular i-stems.

Two-termination adjectives

These share one form for masculine and feminine, with a separate neuter form. The pattern is -is, -e:

• fortis, forte (brave)
• gravis, grave (heavy, serious)
• omnis, omne (all, every)

This is the most common type of third declension adjective. They follow i-stem declension throughout.

One-termination adjectives

These use a single nominative singular form for all three genders. You need the genitive to find the stem:

• fēlix, fēlīcis (happy, fortunate)
• ingēns, ingentis (huge)
• audāx, audācis (bold)

Most one-termination adjectives follow i-stem patterns, so the neuter nominative/accusative plural ends in -ia (e.g., fēlīcia) and the genitive plural ends in -ium (e.g., fēlīcium). The ablative singular uses -ī.

Declension patterns

Masculine and feminine patterns

Masculine and feminine nouns share identical endings in every case except the nominative singular. Here's a sample declension of rēx, rēgis (m., "king"):

Neuter patterns

Here's a sample declension of corpus, corporis (n., "body"):

Notice how the nominative and accusative are identical in both singular and plural, and the plural nominative/accusative ends in -a. For a neuter i-stem like mare, maris ("sea"), the plural nominative/accusative would be maria, the genitive plural marium, and the ablative singular marī.

Common vocabulary

Frequently used nouns

• homō, hominis (m.) human being
• cīvitās, cīvitātis (f.) state, citizenship
• tempus, temporis (n.) time
• lēx, lēgis (f.) law
• mare, maris (n.) sea [i-stem]
• pars, partis (f.) part [i-stem]
• nox, noctis (f.) night [i-stem]
• corpus, corporis (n.) body

Frequently used adjectives

• fēlix, fēlīcis happy, fortunate [one-termination]
• omnis, omne all, every [two-termination]
• fortis, forte brave, strong [two-termination]
• gravis, grave heavy, serious [two-termination]
• brevis, breve short, brief [two-termination]
• ingēns, ingentis huge, enormous [one-termination]
• vetus, veteris old, ancient [one-termination; consonant stem]
• audāx, audācis bold, daring [one-termination]

Translation exercises

Noun-adjective agreement

When a third declension adjective modifies a noun from a different declension, the adjective and noun won't look alike, but they must still agree in gender, number, and case. Practice with phrases like:

• in marī altō ("in the deep sea"): marī is ablative singular of the i-stem neuter mare; altō is ablative singular of the 2nd declension adjective altus
• cum mīlite fortī ("with a brave soldier"): mīlite is ablative singular of mīles; fortī is ablative singular of the i-stem adjective fortis
• cīvēs omnēs ("all the citizens"): both are nominative/accusative plural, third declension

Sentence structure with third declension

Practice identifying third declension nouns in different syntactic roles:

• Subject: Rēx lēgem scrīpsit. ("The king wrote a law.") Both rēx and lēgem are third declension.
• Direct object: Mīlitēs urbem cēpērunt. ("The soldiers captured the city.")
• Prepositional phrase: Prō pāce pugnāvērunt. ("They fought for peace.")

Combining third declension nouns with verbs in different tenses and with nouns from other declensions is the best way to build fluency with these forms.