Final answer:
The ensign, a flag indicating nationality, is flown from the stern of a ship at sea. It is a symbol of identity and pride used historically by vessels like the USS Constitution and the USS Constellation.
Step-by-step explanation:
When a ship is at sea, the ensign is traditionally flown from the rear of the vessel, which is known as the stern. The ensign is a flag that indicates the ship's nationality and is commonly used when a ship is in waters governed by international law. Historical examples of ensign usage can be drawn from warships such as the USS Constitution, also known as "Old Ironsides," and the USS Constellation which displayed their ensigns during naval engagements. The ensign serves not only as a marker of identity but also as a symbol of pride and morale for a ship's crew and country.