[STEMS IN -i- AND MIXED STEMS.]
The Third Declension.
Genitive singular -is, genitive plural -i-um.
[513]. Stems in -i- include both substantives and adjectives, gender words and neuters.
For the gender of substantives, see [570].
[514]. The nominative of gender stems in -i- ends usually in -s (or -x), sometimes in -l or -r; that of neuter substantives has no suffix, and ends usually in -e, sometimes in -l or -r.
[515]. Most stems in -i- have as many syllables in the nominative as in the genitive.
Such words are called Parisyllabic words, or Parisyllables: as, nominative cīvis, citizen, two syllables; genitive cīvis, of a citizen, also two syllables.
[516]. Stems in -i- are declined in the main like consonant stems, but have -im in the accusative of some substantives, and -ī in the ablative of adjectives, of some gender substantives, and of neuters; in the plural they have -ium in the genitive, -īs often in the accusative of gender words, and -ia in the nominative and accusative neuter.