Abstract: A valve including an at least partially spherical valve control member and a valve seat where the valve control member is rotatable within a valve housing to obturate and to open at least one opening of the housing by bringing a passage through the valve control member out of and into alignment with the opening. The valve seat includes a radially outer disc portion which lies in a plane transverse to an axis of the opening and a radially inner generally frusto-conical, annular extension free at its narrower-diameter end. The frusto-conical extension is adapted to lie in the opening with a larger-diameter end inward of the housing, and to make substantially a line contact with an at least partially spherical surface of the valve control member when the latter obturates the opening.