a plastic screw cap has a threaded top portion and tabs of varying heights disposed around an inner circumference of a lower sleeve - portion . these tabs clasp a land disposed on a container neck to restrain the sleeve portion on the neck when the cap is unscrewed . a threaded top portion mates with threads on the container above the land . a sealing liner and diffusion barrier disposed at the bottle opening , together with a crush - resistant cap structure , prevent wine leakage and control oxygen ingress . typically , the plastic cap is threaded onto the container neck , forming a tamper - evident seal . when unscrewed , the plastic - cap top moves up the container threads , exerting a pull on the sleeve portion , which is restrained due to the closure tabs acting on the land . between the closure tabs and the threaded top is a breakaway line , which tears due to the removal forces . this tear evidences tampering .