RF flanges seal with a flat gasket, formerly made of asbestos but now made of more environmentally friendly material, designed for installation between the raised faces of two mating flanges (both with raised faces). The raised faces have a prescribed texture to increase their gripping and retaining force on this flat gasket. Some users of raised face flanges specify the use of spiral wound gaskets. These gaskets may often have an inner ring on their I.D. and an outer disc around their O.D. This feature will hold the gasket concentric with the bolt circle and bore to make precise assembly easier with the gasket centered on the flange sealing surfaces. See an explanation of Spiral Wound Gaskets below.