Accent.—Stems of one syllable accent the case ending in the genitive and dative of all numbers; and -ων and -οιν take the circumflex accent. Thus, φλέψ
a. Exceptions. The ending of the gen. dual and pl. is not accented in the case of ὁ, ἡ παῖς
b. A trisyllabic form, if contracted, does not show the accent on the case ending: ἦρ-ος for ἔαρ-ος, ἦρ-ι for ἔαρ-ι, from τὸ ἔαρ
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].