Entities may also have relationships to other Entities. For example, one may have one Entity to represent cigarette packs at a serial number level, meaning the instances of the Entity correspond to individual packs of cigarettes in the real world and a second Entity to represent individual carton boxes into which the cigarette packs are placed. If one wished to keep track of which cigarette packs were placed in which carton, they would create a relationship from the cigarette pack Entity to the carton Entity. Depending on how one wished to determine what cigarette pack is in which carton, they may define the relationship from pack to carton, carton to pack or in both directions.