O-rings are closed rings of circular cross-section. They are made through vulcanisation from form tools as a complete ring. The sealing function of the O-ring is achieved by the deformation of the cross-section when installed and compressed in its housing. The O-ring can be compressed either radially or axially in its installation housing. In operating conditions the pressure exerted by the media reinforces the sealing function, as under pressure the elastomer material functions as a non-compressible fluid.