Grassman's Law.

Grassman's Law.

From: Comparative Indo-European Linguistics: An Introduction, by Robert S.P. Beekes (John Benjamins, 1995), p.128.
 

Grassman's Law says that an aspirated sound loses its aspiration when followed by another aspirated sound; two aspirated sounds in one form, then, could not be found together. In Sanskrit, for example, dh - dh became d - dh.