When a short ultima of the nominative is lengthened in an oblique case
a. a proparoxytone becomes paroxytone: θάλαττα θαλάττης, ἄνθρωπος ἀνθρώπου.
b. a properispomenon becomes paroxytone: μοῦσα μούσης, δῶρον δώρον.
c. an oxytone becomes perispomenon in the genitive and dative of the second declension: θεός θεοῦ θεῷ θεῶν θεοῖς.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].