Dahl's law

Dahl's law is a sound rule of Northeast Bantu languages, a case of voicing dissimilation. In the history of these languages, a voiceless stop, such as /p t k/, became voiced (/b d ɡ/) when immediately followed by a syllable with another voiceless stop. So, for example, Nyamwezi has -datu "three" where Swahili, a Bantu language which did not undergo Dahl's law, has -tatu, and Shambala has mgate "bread" where Swahili has mkate. Dahl's law is the reason for the name Gikuyu, when the language prefix normally found in that language is ki-.

The law was named in 1903 by Carl Meinhof after the missionary Edmund Dahl, who had discovered it.

Dahl's law is often portrayed as the African equivalent of Grassmann's law in Indo-European languages. However, an analogue of Grassmann's law (which is aspiration, not voicing, dissimilation) has taken place in the Bantu language Makhuwa, where it is called Katupha's law.