Both the objects used to make impressions and the impressions themselves are referred to as seals. Seal impressions served as a signature of the owner of the seal. Seals used to make impressions include cylinder seals and stamp seals. Often these seals are holed for stringing and some were probably never used to make impressions, but were rather worn as amulets. The most common form of seal impression is the bulla. A bulla (plural, bullae), is a lump of clay or lead molded around a cord and stamped with a seal that identifies the sender. With a bulla in place a container cannot be violated without visible damage to either the bulla or the cord, thereby ensuring the contents remain tamper-proof until they reach their destination.