From: P&G
Message: 31346
Date: 2004-03-03
>The pronoun *ís, *ésyo gives Latin is, e:ius. Final -s isNot quite right. The vowel was short, and the medial <-i-> was pronounced
>secondary, so the form reflects stressed *ésyo with loss of
>-s- and compensatory lengthening of the /e/. With stressed
>*-ósyo the result is -u:ius (*-ósyo > *-óhyo > *-ó:io >
>*-ú:iu + s), as in hu:ius, cu:ius.