A closure 10 for a container, comprises a substantially circular base 20, a skirt 30 extending from the periphery thereof, a bore seal 50 in the form of an annulus and a rib 70 lying on the surface of the base 20 in a substantially radial direction and in contact with the radially inner surface of the bore seal 50 at one end for transferring any movement of the center of the base 20, relative to the skirt 30, to the bore seal 50 such that the bore seal 50 is pulled radially inward to allow venting of excess pressure within the container, wherein the end of the rib 70 in contact with the radially inner surface of the bore seal 50 is substantially thinner, in a plane parallel to the base 20, than the other end of the rib 70.