Boatswain's call

A boatswain's call, pipe, or bosun's whistle is a pipe or a non-diaphragm type whistle used on naval ships by a boatswain.

The pipe consists of a narrow tube (the gun) which directs air over a metal sphere (the buoy) with a hole in the top. The player opens and closes the hand over the hole to change the pitch. The rest of the pipe consists of a "keel", a flat piece of metal beneath the gun that holds the call together, and the "shackle", a keyring that connects a long silver or brass chain that sits around the collar, when in ceremonial uniform.