An **ensign at sea** is the national or service flag flown by a vessel to declare its nationality and, in the case of warships, its service. When at sea, the ensign is flown at the gaff or, if no gaff is fitted, at the masthead; when at anchor or alongside, it is flown at a staff at the stern. Ensigns are hoisted at colours and lowered at sunset in keeping with the sunrise-to-sunset rule.
**Example**: A merchant ship at sea displays her national ensign at the gaff and a courtesy flag of the host country at the foremast when in foreign waters.
**See also**: [ensign](/glossary/ensign/), [jack-at-bow](/glossary/jack-at-bow/), [courtesy-flag](/glossary/courtesy-flag/), [sunrise-to-sunset-rule](/glossary/sunrise-to-sunset-rule/)