During the Byzantine period, lead bullae (singular, Bulla) were widely used to seal and identify the sender of correspondence and containers in shipment. An iron, pliers-shaped instrument, a boulloterion, was used to impress the designs on a lead bulla seal. After the cord was wrapped around the package or document and the ends inserted in a channel in the blank seal, the seal was placed between the disc shaped engraved dies on the jaws of a boulloterion. The boulloterion had a projection above the jaws, which was struck with a hammer to impress the design on the seal and close the channel around the two ends of the cord. With a bulla in place a container cannot be violated without visible damage to either the bulla or the cord, ensuring the contents remain tamper-proof until they reach their destination.