33 CFR 23.05 - Where and when displayed.
(a) The Coast Guard Ensign is a mark of authority and is required to be displayed whenever a Coast Guard vessel takes active measures in connection with boarding, examining, seizing, stopping or heaving to of a vessel for the purposes of enforcing the laws of the United States. The distinctive markings of Coast Guard aircraft serve the same purpose.
(b) The Coast Guard Commission pennant indicates a Coast Guard cutter under the command of a commissioned officer or commissioned warrant officer.
(c) When applicable, these distinctive marks shall be displayed, the Coast Guard Ensign at the masthead of the foremast, and the commission pennant at the after masthead. On ships having but one mast the Coast Guard Ensign and commission pennant shall be at the masthead on the same halyard. In mastless ships they shall be displayed from the most conspicuous hoist.