Third-Declension Three-Termination Adjectives

A three-termination third-declension adjective has a different nominative singular form for each gender: ācer masculine, ācris feminine, ācre neuter, “sharp, fierce, keen.” The three-way distinction exists only in the nominative and vocative singular. Everywhere else masculine and feminine share a form, and often all three genders do.

This class is small, but its pattern introduces the i-stem endings used throughout third-declension adjectives: ablative singular -ī, genitive plural -ium, and neuter plural -ia. “Three-termination” describes forms, not three meanings and not three separate adjectives. Compare the different nominative organization of Two-Termination Adjectives.

The complete paradigm of ācer, ācris, ācre

The stem is ācr-, revealed by feminine ācris and neuter ācre. The masculine nominative inserts e before final -r: ācer, not ❌ ācr. The long ā belongs to the lexical stem and remains throughout the paradigm.

The three nominative singulars

Masculine nominative singular ācer agrees with ventus. The adjective’s sense is “fierce” or “strong,” not literally “sharp”; the head noun guides the idiomatic English rendering.

Feminine nominative singular ācris agrees with hiems. English leaves “bitter” unchanged, while Latin distinguishes the feminine from masculine ācer.

Neuter accusative singular ācre agrees with direct object vīnum. Because neuter nominative and accusative are identical, only syntax shows that the phrase is the object of bibere.

An authentic poetic line is reproduced without added macrons:

Here acris is the ordinary feminine nominative with hiems. The line is (literary) poetry, but the agreement itself is standard Classical morphology.

Shared singular forms beyond the nominative

Masculine and feminine accusative singular share -em; all genders share genitive -is, dative -ī, and ablative -ī. Neuter accusative remains identical to neuter nominative.

Masculine accusative ācrem agrees with ōrātōrem. Agreement does not reveal masculine by the adjective alone here; the noun’s gender and meaning do.

Ablative singular ācrī agrees with masculine dolōre. The noun uses consonant-stem ablative -e, while the adjective uses i-stem -ī: dolōre ācrī. Their endings differ, but their features match.

Both febrī and ācrī are feminine ablative singular after dē. The surface match is convenient but not required, as dolōre ācrī showed.

The i-stem signatures in the plural

The most diagnostic endings are genitive plural -ium and neuter nominative/accusative plural -ia.

Masculine nominative plural ācrēs agrees with ventī. Feminine accusative plural parvās belongs to a different adjective class and agrees with nāvēs.

Neuter plural verba requires ācria, not masculine/feminine ācrēs. The finite verb is plural laedunt, as normal with a neuter plural subject in Latin.

Genitive plural ācrium agrees with dolōrum. The noun has second-declension -ōrum while the adjective has third-declension i-stem -ium.

Masculine and feminine accusative plural ācrīs is an inherited Classical alternative to ācrēs. It is common in some authors and editorial traditions, especially in (literary) texts. For straightforward active prose, ācrēs is a safe regular choice; for reading, recognize both. This variation does not affect neuter ācria.

Other three-termination adjectives

The same structural type includes celer, celeris, celere “swift” and volucer, volucris, volucre “flying, swift.” Their stems must be learned from the feminine and neuter forms, just as with ācer.

Masculine celer agrees with nūntius; feminine celeris agrees with nāvis. Neuter would be celere animal. The masculine does not provide enough information by itself to generate every form reliably, so retain the three-part dictionary entry.

This is not a large productive class into which new adjectives are freely placed. Most third-declension adjectives have two terminations (fortis, forte) or one termination (fēlīx, fēlīcis). Those classes preserve much of the same i-stem morphology but organize the nominative singular differently.

Attributive, predicative, and positioned for focus

Like other adjectives, ācer may be attributive or predicative:

Attributive ācris belongs to ōrātiō. In ōrātiō ācris est, the same form would be predicative. Position after the noun can make the quality more descriptive or contrastive, but it does not alter agreement. (Literary) authors may separate adjective and noun widely for framing or sound; the shared feature bundle still connects them.

Older Latin and poetry occasionally preserve nonstandard gender forms, including masculine ācris or feminine ācer. These are (archaic/literary) variants, not forms to mix into the neutral Classical paradigm above.

A parsing and production method

1. Identify the head noun’s lexical gender.
2. Determine its number and case from syntax.
3. If the adjective is nominative/vocative singular, choose ācer, ācris, or ācre by gender.
4. Otherwise select the shared case form: for example, accusative masculine/feminine ācrem but neuter ācre.
5. Check the i-stem diagnostics: ablative singular -ī, genitive plural -ium, neuter plural -ia.

Common Mistakes

1. Using the masculine nominative with a feminine noun

2. Remodeling the neuter on bonus, bona, bonum

3. Forgetting neuter plural -ia

4. Replacing i-stem ablative -ī with noun-like -e

Key Takeaways

Three-termination adjectives distinguish masculine, feminine, and neuter only in the nominative and vocative singular. Ācer, ācris, ācre uses stem ācr- and the i-stem markers -ī, -ium, and neuter plural -ia. Learn all three citation forms, agree by grammatical features rather than visible endings, and treat archaic gender variants as reading forms rather than neutral prose models.