biu-mandara: a group of Chadic languages spoken in the border area between Cameroon and Nigeria south of Lake Chad
chad: a family of Afroasiatic tonal languages (mostly two tones) spoken in the regions west and south of Lake Chad in north central Africa
stratum: one of several parallel layers of material arranged one on top of another (such as a layer of tissue or cells in an organism or a layer of sedimentary rock)