The ensign of the United States refers to the flag of the United States when worn as an ensign (a type of maritime flag identifying nationality, usually flown from the stern of a ship or boat). International maritime law—see International Treaty on Law of the Sea, articles 91 and 92—provides that vessels have a national character and thus should display a flag (ensign) that corresponds to this national character, especially when in international or foreign waters. Vessels that are formally documented under the federal vessel documentation act, vessels owned by government bodies in the United States, and vessels in the US armed forces unquestionably have US national character, and thus properly hoist a U.S. ensign to show their national character. Vessels that are numbered by the states and small, non-registered craft owned by American citizens may also hoist a U.S. ensign to show their national character.
Some other flags are often used as “civilian” or “yacht” ensigns in place of the national flag by smaller vessels when operating within US waters; this practice arises from longstanding historical use and custom. All vessels of U.S. national character should display the national ensign when operating in international and non-US waters.