This paper presents a formal definition of the Tansill formation and detailed measurement and description of the section exposed at the type locality. The typical surface section is correlated with well known subsurface sections in order that the application of the name in the subsurface may be clearly understood. In addition the relation of the Tansill limestone to the Tansill anhydrite and the relations of the Tansill formation to the Capitan limestone, and to the underlying Yates and overlying Salado formations, are briefly set forth. The Tansill formation is bounded by two important Permian horizons of West Texas and New Mexico: at the base is the familiar "top of the Yates," and at the top is the boundary between the Ochoa and Guadalupe series. These are designated T and OG, respectively, and some significant facts about them are brought out. The Ocotillo silt member of the Tansill formation is also named and defined.