Allen and Greenough's New Latin Grammar for Schools and Colleges
J. B. Greenough, G. L. Kittredge, A. A. Howard, Benj. L. D'Ooge, Ed.

Of these the singular is thus declined:—

[*] a. The plural of these words is regular, like that of bonus (§ 110).

[*] b. The genitive in -īus, dative in -ī, and neuter in -d are pronominal in origin (cf. illīus , illī , illud , and § 146).

[*] c. The i of the genitive ending -īus, though originally long, may be made short in verse; so often in alterius and regularly in utriusque .

[*] d. Instead of alīus , alterīus is commonly used, or in the possessive sense the adjective aliēnus, belonging to another, another's.

[*] e. In compounds—as alteruter—sometimes both parts are declined, sometimes only the latter. Thus, alterī utrī or alterutrī, to one of the two.

[*] Note.--The regular genitive and dative forms (as in bonus ) are sometimes found in some of these words: as, genitive and dative feminine, aliae; dative masculine, aliō . Rare forms are alis and alid (for alius, aliud).