Introduction
Third Declension Latin Nouns
Mixed i-stems are a subset of Third Declension nouns that largely decline like standard Third Declension nouns but have a Genitive Plural ending in -ium, like i-stem nouns.
There are no hard and fast rules for distinguishing mixed i-stems from pure i-stems, but some guidelines exist.
Some masculine and feminine mixed i-stems have the same number of syllables in the Nominative Singular and the Genitive Singular.
Masculine mixed i-stems end in -is in the Nominative Singular, while feminine mixed i-stems can end in -is or -es in the Nominative Singular.
Some Latin nouns are only used in the plural.
Examples of such nouns include menses, nares, meninges, and leptomeninges.
Leptomeninges has a Genitive Singular of piae matris, while pachymeninx, which means "dura mater," has a Genitive Singular of durae matris.
The Imparisyllabic Rule, sometimes called the Double-Consonant Rule, applies to Third Declension nouns of all genders.
Nouns which are monosyllabic in the Nominative Singular and end in -s, usually preceded by a consonant such as p, n, or r, belong to this group.
These nouns frequently decline by replacing the -s with -t and feature two consonants before the -is of the Genitive Singular.
In English, these words often share the same stem as the Latin.
For example, the Latin word pars, meaning "part," has the Genitive Singular partis.