I have three older stainless steel pressure cookers, all Presto. 3, 4 and 6 qt. Same gasket for all. It is, as PrincessFiona60 says, "U" shaped in profile, with the open face of the "U" facing inward. As the cooker pressurizes, it expands the gasket upward, downward and outward as far as the lip of the lid will permit, sealing the cooker. Most pressure cookers also have a safety relief that will blow if the cooker over-pressurizes. Else it could become a bomb.