The bosun's (or boatswain's) call, also called a whistle, has traditionally been used to signal and command sailors.

By varying the force used to blow into the call, and by using fingers to control the amount of air escaping from the top of the spherical "buoy" (left), different notes can be sounded. Various combinations of notes can be used to issue different commands.

Bosun's Call
CWM 20070025-015