Allen & Greenough · Part First — Words and Forms › THIRD DECLENSION (CONSONANT AND i-STEMS)

§71. Mixed i-Stems

Mixed i-stems have -em in the accusative and -e in the ablative singular, -ium in the genitive¹ and -īs or -ēs in the accusative plural. They include the following:—

Nouns in -ēs, gen. -is.²
² These are acīnacēs, aedēs, alcēs, caedēs, cautēs, clādēs, compāgēs, contāgēs, famēs, fēlēs, fidēs (plural),indolēs, lābēs, luēs, mēlēs, mōlēs, nūbēs, palumbēs, prōlēs, prōpāgēs, pūbēs, sēdēs, saepēs, sordēs, strāgēs, struēs, subolēs, tābēs, torquēs, tudēs, vātēs, vehēs, veprēs, verrēs, vulpēs; aedēs has also nominative aedis.
Monosyllables in -s or -x preceded by a consonant: as, ars, pōns, arx.
Polysyllables in -ns or -rs: as, cliēns, cohors.
Nouns in -tās, genitive -tātis (genitive plural usually -um)³: as, cīvitās.
³ There is much variety in the practice of the ancients, some of these words having -ium, some -um, and some both.
Penātēs, optimātēs, and nouns denoting birth or abode (patrials) in -ās, -īs, plural -ātēs, -ītēs: as, Arpīnās, plural Arpīnātēs;Quirīs, plural Quirītēs.
The following monosyllables in -s or -x preceded by a vowel: dōs, fraus, glīs, līs, mās,mūs, nix, nox, strix, vīs.

J. H. Allen and J. B. Greenough, New Latin Grammar (Boston: Ginn, 1903). Text provided under a CC BY-SA license by Perseus Digital Library, http://www.perseus.tufts.edu, with funding from the National Endowment for the Humanities.