A flag is flown at half-mast (or half-staff) as a symbol of respect, mourning, distress, or as a salute. The tradition dates back to the 17th century and is often used to honor the memory of deceased prominent figures, mark national days of mourning, or commemorate tragic events. Protocol typically requires that the flag be initially raised to the top of the pole and then lowered to the half-mast position, leaving a space for an "invisible flag of death" above it. For example, in the United States, the president can issue an order to fly the flag at half-staff upon the death of major government figures or during periods of national mourning. Flags at half-mast are also common following tragic incidents or to honor fallen service members.