Dahl's law is a sound rule in some of the 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. For example, Nyamwezi has -datu "three" where Swahili, a Bantu language that 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. It is productive in Sukuma, in the Nyanyembe dialect of Nyamwezi, most E50 languages (such as Kikuyu, Embu, Meru/Chuka), some J languages (such as Rwanda, Gusii, Kuria). In other languages the law is no longer productive, but there are indications that it once was (such as in Taita, Kamba/Daisũ, Taveta, Luhya/Logoli). In some neighboring languages (and in other dialects of Nyamwezi) words reflecting Dahl's law are found, but they appear to be transfers from languages in which the law is productive.
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.
- BFYP Masele & Derek Nurse (2003) "Stratigraphy and prehistory: Bantu Zone F". In Henning Andersen (ed.) Language Contacts in Prehistory: Studies in Stratigraphy. John Benjamins.