The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
77. The regular Nominative plural of i-stems is -és,[The Indo-European ending of the
nominative plural, -es (preserved in Greek consonant stems, as o, o-), contracts with a
stem-vowel and gives -és in
Latin i-declension (cf. the Greek plural o). This -és was extended to consonant stems in
Latin.] but -ís occasionally
found. The regular Accusative plural -ís is common but not exclusively used
in any word. An old form for both cases is -eis (diphthong).