A closure for covering an opening of a container. The closure includes an inverted U-shaped peripheral channel for receiving the rim of a container therein. A tear strip is formed with the skirt of the closure via a tear line that circumscribes the skirt. A breakaway pull tab having an inner surface with U-shaped ribs, the combination of the surface and ribs being such as to provide a concavity which assists in gripping the tab to pull the strip away. Linear vertical ribs may be provided on the outer surface to provide further assistance in the gripping function. The skirt includes a recessed area underlying the pull tab for providing access to the pull tab. An alternative embodiment uses a zig-zag structure of strengthening ribs in the skirt to reduce the amount of material necessary to provide the undercut. This reduces the probability of warpage in the closure structure.