A pack or packet of cigarettes (also informally called fag packet in British slang; as in the idiom "back of a fag packet" or "fag-packet calculation") is a rectangular container, mostly of paperboard, which contains cigarettes. The pack is designed with a flavor-protective foil, paper or plastic, and sealed through a transparent airtight plastic film. By pulling the "pull-tabs", the pack is opened. Hard packs can be closed again after opening, whereas soft packs cannot. A patent has been granted for a cigarette package containing a container for disposal of cigarette butts.