Proto-Balto-Slavic, the ancestor of Proto-Slavic, possessed three vocalic diphthongs: *ai, *au and *ei. In Proto-Slavic, these were monophthongized as follows, with the subscript indicating whether the vowels trigger the first palatalization or the second.

* *ai > *e2, *i2 - Early Slavic *snaigu > *sneg? > Serbo-Croatian snijeg
* *au > *u - Early Slavic *sausu > *sux? > Serbo-Croatian