© 2018 National Association of Professors of Hebrew. All Rights Reserved. This article is the first half of a study, continued in the next volume of Hebrew Studies. The 3 m. sg. pronominal suffix on plural nouns is realized in several allomorphs in Classical Hebrew: In early Hebrew inscriptions, the suffix appears as <-W> and perhaps as <-YH>; in Biblical Hebrew, it usually appears as <-YW> (sometimes emended from <-W> in instances of Masoretic Qere readings), and occasionally as <-YHW> in poetic texts. In this study we provide a unified and principled linguistic account of these textual data, tracing the various phonological developments of the 3 m. sg. genitive suffix on plural nouns, and relating these phonological developments to the phonetic causes underlying them. After analyzing the phonological realizations of the high vocoids ∗U (/w/ and /u/) and ∗Y (/y/ and /i/) and of ∗H (found in the third-person pronominal morphemes), we identify three stages of development that produced the <-Y-> in Biblical Hebrew: (1) the linkage of the number-gender morpheme to a single slot in the skeletal tier (effectively yielding an early diphthong contraction ∗-aI > ē); (2) the deletion of ∗H in selected environments defined by accent and the surrounding vowels; and (3) the phonetically-motivated insertion of the glide ∗y in the hiatus environment [-e:w:].