Abstract: A rotary plug valve is disclosed in a preferred embodiment that has two telescoping seat assemblies that fit into respective recesses provided in the valve body surrounding a passageway through the plug valve. A valve chamber is formed in the valve body into which the rotary plug may be positioned to control flow through the valve by rotation thereof. In a preferred embodiment, the telescoping seat assemblies provide an upstream and a downstream seal with the rotary plug. The telescoping seat assemblies preferably function differently when in the open and closed position so that both assemblies expand when in the open position whereas in the closed position one assembly expands and one is compressed A line pressure surface is provided to provide a force against the plug seal seat element. The plug seal seat element and plug mate with a curved surface and the contact stress varies depending on the location along the curved surface.