F. Sommer, in the revised edition of his Handbuch der lateinischen Laut- und Formenlehre, offers no explanation of the development of final -āi in the dative singular of ā-stems (= first declension) to -ae rather than to ī; see pp. 146, 327–28. Yet the development of the long diphthong āi to ī in non-initial (= unaccented) syllables is certain in the dative-ablative plural of ā-stems, and an identical development of the long ōi is seen in the same forms of the o-stems, where length is assured by the Sanskrit form: inst. pl. devāis = Latin dīvīs. The long diphthong in the dative singular of ā-stems is proved by the Greek ending in ‘terrae,’ ‘deae.'