Jamsay is the largest-population language among some twenty Dogon languages in Mali, West Africa. This is the first comprehensive grammar of any Dogon language, including a full tonology. The language is verb-final, with subject agreement on the verb and with no other case-marking. Its most striking feature is the morphosyntactically triggered use of stem-wide tone-contour overlays on nouns, verbs, and adjectives. All stems have a lexical tone contour such as H[igh], L[ow]-H, HL, or LHL with at least one H-tone. An exam of tone overlay is tone-dropping to stem-wide all-L. This is used for Perf.