Private signal

Private signals are custom designed flags used to symbolize and identify the owner of a boat. These generally have a swallowtail shape but may instead be rectangular or sometimes triangular. Private signal tradition is drawn from heraldry but typically does not incorporate intricate designs from a family crest.

A private signal may be hoisted while underway and at anchor, day or night, but not while racing.Power boats fly the owners' private signal at the top of the main-mast head or from a short staff on the bow called a bow staff. On a sailboat the private signal is flown using a pig stick hoisted to the top of the main-mast or mizzen-mast.

From as far back as 4000 BC, Egyptian captains of the Nile would identify themselves to passing ships by placing a clay figurine atop their ships’ aftermost cabin.

The Romans used private signals quite often. At sea off Marseilles in 49 BC, the famous Roman Brutus hoisted his private signal during a battle against the Massilians of Pompeii. Primary documents say the Massilians recognized his “flamboyant” signal, so his ship narrowly escaped being rammed simultaneously by two triremes.

In the Middle Ages private signals were used extensively at sea. The Bayeux Tapestry of 1150 AD shows “Mora,” the ship of William, Duke of Normandy, as it flew a white banner, bordered in blue and bearing a golden cross. Other knights in the tapestry boast their own private signals.

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Wikipedia