Third Declension (rex, regis)
The hardest and most common declension. Learn to identify the genitive stem, the i-stems, and the patterns that unlock most Latin nouns.
A lesson of Latin A1 — Beginner, in Noun Declensions, about 10 minutes.
By the end of this lesson, you will identify third-declension nouns by their genitive form, master the case endings, and read Latin sentences containing these extremely common nouns.
Why the Third Declension Is Central
The third declension is the largest and most important in Latin. It contains nouns of all three genders — masculine, feminine, and neuter — and includes many of the most common Latin words: rex (king), lex (law), homo (man), nox (night), pax (peace).
There is no single nominative ending that marks a third-declension noun. Instead, you recognize them by their genitive singular ending: -is.
The golden rule: when you see a new Latin noun, look at the genitive form. If it ends in -is, it is third declension.
The Genitive Reveals the Stem
Unlike first and second declension, the nominative and genitive of third-declension nouns often look very different. The genitive singular gives you the stem you add all other endings to.
| Nominative | Genitive | Stem | Meaning |
|---|---|---|---|
| rex | regis | reg- | king |
| lex | legis | leg- | law |
| nox | noctis | noct- | night |
| homo | hominis | homin- | man/person |
| corpus | corporis | corpor- | body |
| caput | capitis | capit- | head |
Technique: always memorize a new noun with both forms: rex, regis — never rex alone.
Third-Declension Endings
Masculine and Feminine (same endings)
| Case | Singular | Plural |
|---|---|---|
| Nominative | — (varies) | -ēs |
| Genitive | -is | -um |
| Dative |