Proto-Nuristani is the latest common form of all modern-day Nuristani languages.
The earliest divergence of Proto-Nuristani from the other Indo-Iranian languages may be indicated by the fact that the Ruki sound law does not apply after *u: e.g. Southeastern Katë musë /muˈsɘ/ "mouse" < *mūs-a-ka-.
[1] Proto-Nuristani shares with Iranian languages the merger of the tenuis and breathy-voiced consonants, the preservation of the distinction between the two sets of Indo-Iranian voiced palatals (which merged in Indo-Aryan), and the fronting of the Proto-Indo-Iranian primary palatal consonants.
The latter were retained as dental affricates in Proto-Nuristani, in contrast to simplification to sibilants in most Iranian languages.
[2] This is demonstrated by the existence of disparate sound changes in individual Nuristani languages that make it difficult to neatly classify them into groups that share a common sound change on the same timeline.