A boatswain's call, pipe or bosun's whistle is a chain. It is pronounced, and sometimes spelled, "bosun's call".

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.

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