A type of aid to navigation, a buoy with a conical top, resembling the wimples once worn by some orders of nuns.

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Nuns are cheaper than full-blown lighted buoys, and in the United States, they are used to mark the starboard side of the channel when entering a small harbor (which becomes the port side when leaving). In these cases a nun buoy is painted red, and has an even number on it. When there is a preferred channel, green-and-red striped nuns with a red stripe at the top will mark the port side, entering.

Nuns are also frequently used to mark underwater obstructions, moorings, and anchorages, or the ends of measured distance lines.