Fifth Declension
Fifth declension is the smallest family in Latin — stems in -ē-, nominative -ēs, genitive -ēī (or -eī after a consonant). Almost every member is feminine.
The whole declension would be a footnote except that two of its nouns, rēs ("thing, matter, affair") and diēs ("day"), are among the most-used words in the language.
Rēs pūblica, rēs gestae, in diēs, hodiē — you cannot read a page of Cicero or Caesar without bumping into a fifth-declension form.
Three small things to learn and you own this declension. Diēs is the gender outlier: usually masculine, but feminine when it names a single appointed day or time-in-general (cōnstitūtā diē).
Most fifth-declension nouns are defective in the plural — only rēs and diēs run the full table. And the genitive -eī is the tiebreaker against third-declension -ēs nouns (nūbēs, nūbis).
Stem in -ē-; nominative -ēs. Almost every noun is feminine; diēs and merīdiēs are masculine.
Only rēs and diēs have a full plural — most others are singular-only. Genitive -eī is the tiebreaker against 3rd-declension -ēs nouns.
| Case | Singular | Plural |
|---|---|---|
| Nom. | r-ēs | r-ēs |
| Gen. | r-eī | r-ērum |
| Dat. | r-eī | r-ēbus |
| Acc. | r-em | r-ēbus |
| Abl. | r-ē | r-ēbus |
| Case | Singular | Plural |
|---|---|---|
| Nom. | di-ēs | di-ēs |
| Gen. | di-ēī | di-ērum |
| Dat. | di-ēī | di-ēbus |
| Acc. | di-em | di-ēs |
| Abl. | di-ē | di-ēbus |
| Loc. | di-ē | — |
| Case | Singular | Plural |
|---|---|---|
| Nom. | fid-ēs | — |
| Gen. | fid-eī | — |
| Dat. | fid-eī | — |
| Acc. | fid-em | — |
| Abl. | fid-ē | — |
See It In Action
— B. G. i. 30. 5
Both fifth-declension stars in one sentence: rē (abl. sg. of rēs) inside an ablative absolute, and diem (acc. sg. of diēs) — feminine here because it names a fixed appointed day.
— Sall. Cat. 10. 1
Rēs pūblica — literally "the public thing" — is a fixed phrase you'll meet on every other page of Roman political prose. Both words are 5th-declension nominative.
— Sall. Cat. 20. 6
In diēs ("day by day") is a fossilized accusative-plural idiom that uses diēs as a unit of time. You'll meet it constantly in narrative prose alongside cotīdiē and hodiē.
Both declensions have nominatives in -ēs. The genitive is the only reliable tell.
gen. sg. -ēī / -eī
rēs, reī
thing — gen. "of the thing"
gen. sg. -is
nūbēs, nūbis
cloud — gen. "of the cloud"
Tip: Always check the genitive in your dictionary headword. -eī = 5th. -is = 3rd. The nominative alone never decides.
In Eā rē permissā, diem conciliō cōnstituērunt (B. G. i. 30. 5), what gender is diem and why?
Study Tips
- •Drill rēs and diēs cold — they are the only two with a full plural, and they cover 90% of fifth-declension Latin you will ever read.
- •When you spot a noun ending in -ēs, check the genitive: -eī/-ēī means fifth declension; -is means third. The nominative alone never decides it.
- •Treat diēs as masculine by default. Flip to feminine only when context names a fixed date (cōnstitūtā diē, longa diēs) — then go back to masculine.
- •Memorize the locative hodiē ("today") and prīdiē ("the day before") as fossilized words, not paradigm forms — that's all the locative does in this declension.