a one - piece plastics closure for a carbonated beverage bottle has a sealing ring which extends inwardly and downardly towards the closure mouth from its attachment to the closure body . when the closure is fitted the ring extends around the outer radius and over the annular top face of the bottle neck , backed by a stepless abutment and reaction surface of the closure body . the latter surface engages the sealing ring mechanically above the container top face to determine the fitted position of the closure and create a seal when the pressure in the bottle is low . when the product pressure is high , however , the effective seal is made between the sealing ring and the outer radius of the bottle neck . for that purpose the abutment and reaction surface is roughened , so allowing access for the product pressure to the back of the sealing ring .